Hmmm.. I wonder... has anyone ever duct-taped or otherwise sealed off their centerboard slot and seen if it makes a significant difference? Perhaps it would be an interesting test....Moe wrote:The 26M is a few hundred pounds heavier when lightly loaded, and the deeper V hull generates less lift, as well as increases drag with greater wetted surface area. Whether the lower drag of a daggerboard vs centerboard slot is enough to offset this (or according to Roger, more than offset it), is questionable IMHO.
New '05 26M Owner - Hoping for some guidance.
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kmclemore
- Site Admin
- Posts: 6263
- Joined: Sun Feb 08, 2004 9:24 am
- Sailboat: MacGregor 26X
- Location: Ambler, PA -- MACX2018A898 w/ Suzuki DF60AV -- 78 BW Harpoon 4.6 -- 2018 Tahoe 550TF w/ 150 Merc
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mike
- Captain
- Posts: 812
- Joined: Sun Dec 28, 2003 8:06 pm
- Location: MS Gulf Coast "Wind Dancer" 98 26X
Good point... and this could spawn a great new mod! You know, some sort of retractable centerboard slot door/cover system.kmclemore wrote:Hmmm.. I wonder... has anyone ever duct-taped or otherwise sealed off their centerboard slot and seen if it makes a significant difference? Perhaps it would be an interesting test....
--Mike
Terry,
The smaller Mercs with the 14" props use the same larger gearcase as the 75-115HP motors, so yes, the hub is larger diameter. I might also note the props for the BigFoots are special silver-painted versions with pressed in very soft rubber hubs to reduce clutch rattle, compared to the multi-piece hubs on other Merc props. However, an increase in the hub diameter, which is small relative to the diameter of the prop, covers much less area than the same increase at the outer diameter of the prop. And the 12" to 14" increase is much larger than that of the hub, so the larger hub doesn't reduce blade thrust anywhere near the amount the larger prop diameter increases it.
The problem with comparing pitch, even between the same manufacturer and even between the same model from the same manufacturer, is that modern props aren't necessarily the same pitch throughout the length of the blade, so the manufacturer has to settle on how far out they want to measure pitch, or how they'll integrate all the different pitches at different blade widths. But that's all we have to work with.
That also throws off slip calculations. For example, in one load configuration, my 60HP BigFoot on the Whaler gets 31.8 mph top speed at only 5200 rpm with the 15" pitch prop. With 2.33:1 gearing, that's -0.13% slip. The 13" prop also gets 31.8 mph top speed at 6,000 rpm, for -0.31% slip. We know negative slip isn't possible, so the assigned pitches are obviously not the same as the effective pitch. That's only two data points in one series of props, and I'm not sure how it applies to the steeper pitches. That difference of 800 rpm also double the 200 rpm per inch of pitch change rule of thumb.
BTW, the first stage of the rev limit is at 6225 rpm, where cylinders 2 and 3 are shutdown. All four go down at 6350.
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Moe
The smaller Mercs with the 14" props use the same larger gearcase as the 75-115HP motors, so yes, the hub is larger diameter. I might also note the props for the BigFoots are special silver-painted versions with pressed in very soft rubber hubs to reduce clutch rattle, compared to the multi-piece hubs on other Merc props. However, an increase in the hub diameter, which is small relative to the diameter of the prop, covers much less area than the same increase at the outer diameter of the prop. And the 12" to 14" increase is much larger than that of the hub, so the larger hub doesn't reduce blade thrust anywhere near the amount the larger prop diameter increases it.
The problem with comparing pitch, even between the same manufacturer and even between the same model from the same manufacturer, is that modern props aren't necessarily the same pitch throughout the length of the blade, so the manufacturer has to settle on how far out they want to measure pitch, or how they'll integrate all the different pitches at different blade widths. But that's all we have to work with.
That also throws off slip calculations. For example, in one load configuration, my 60HP BigFoot on the Whaler gets 31.8 mph top speed at only 5200 rpm with the 15" pitch prop. With 2.33:1 gearing, that's -0.13% slip. The 13" prop also gets 31.8 mph top speed at 6,000 rpm, for -0.31% slip. We know negative slip isn't possible, so the assigned pitches are obviously not the same as the effective pitch. That's only two data points in one series of props, and I'm not sure how it applies to the steeper pitches. That difference of 800 rpm also double the 200 rpm per inch of pitch change rule of thumb.
BTW, the first stage of the rev limit is at 6225 rpm, where cylinders 2 and 3 are shutdown. All four go down at 6350.
--
Moe
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Dimitri-2000X-Tampa
- Admiral
- Posts: 2043
- Joined: Fri Jan 02, 2004 5:36 am
- Sailboat: MacGregor 26X
- Location: Tampa, Florida 2000 Mercury BigFoot 50HP 4-Stroke on 26X hull# 3575.B000
This idea was actually covered in some detail a while ago...but I couldn't come up with anything on a search. Its possible that it was on the old site (pre 2004).Good point... and this could spawn a great new mod! You know, some sort of retractable centerboard slot door/cover system.
There seemed to be a lot of people interested in trying something. Doubt duct tape would hold for long, but some people advocated trying to make a rubber flap. I never heard of anyone actually trying it though.
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ken lockhart
Cover for CB Slot
All, the San Juan 21 sailboat has done that for years, they have a material that will cover the slot when the board is down, I doubt if it would work on the Mac at higher speeds.
Ken
Ken
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Frank C
The theory has been batted-about for years, but no test reported that I've ever seen. Just think about the amount of water that gets jammed into that cavity - kinda like towing a bucket! The context has usually been to improve our back-of-fleet, turkey status. Slot gaskets are not unusual for sail-racing hulls. Duct tape would sure make for a quick test, and I'd bet very effective. Look to improve power-on, WOT by 2 or 3 mph . . . just one opinion.kmclemore wrote:Hmmm.. I wonder... has anyone ever duct-taped or otherwise sealed off their centerboard slot and seen if it makes a significant difference? Perhaps it would be an interesting test....
Frank C (7/05/04) wrote:Tony,
CB trunk has been debated a lot. Somebody actually created an animation of an articulated cover plate that would be pulled down to fill the gap as the board drops. I've theorized a simple vinyl gasket glued to the hull bottom. If your boat is slipped, Phase I would be to drop the board, stuff the gap with foam or styrofoam, and get out for some testing under sail. If it adds a bunch of performance (I think it will) then you can move on to Phase II prototyping.
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Paul S
- Site Admin
- Posts: 1672
- Joined: Tue Jan 06, 2004 10:50 am
- Sailboat: Other
- Location: Boston, MA
- Contact:
Thanks.
It needs updating..but with a foot of snow on the ground....not going to happen too soon.
I like to keep it simple...do what works for us...add things as needed.
Before I drill any holes...I try and do as much research as possible on the topic (like the M main sheet aft thread a couple days ago). Why re-invent the wheel, if someone has done it already?
I installed about 1/3 of the stuff I bought last year. It takes a lot longer to do the installs the way I want...instead of just throwing it in there. Like the battery and inverter install...I actually made the cables to the length needed instead of off the shelf lengths. Fits/looks better..and was far cheaper too. Took a lot longer to do as well. If I think something will take an hr or 2..it usually takes me a day or 2
I would never be a good rigger the customer would never get the boat
It is a bit disappointing the overall quality of the boat and the dealer installed options (or lack thereof). The trailer frame and gelcoat finish is the most disappointing. I do like the disc brakes and the electric lockout for reverse...but I came to accept the Mac for what it is.
More mods have been done and more coming..with lots of pictures to come.
Paul
It needs updating..but with a foot of snow on the ground....not going to happen too soon.
I like to keep it simple...do what works for us...add things as needed.
Before I drill any holes...I try and do as much research as possible on the topic (like the M main sheet aft thread a couple days ago). Why re-invent the wheel, if someone has done it already?
I installed about 1/3 of the stuff I bought last year. It takes a lot longer to do the installs the way I want...instead of just throwing it in there. Like the battery and inverter install...I actually made the cables to the length needed instead of off the shelf lengths. Fits/looks better..and was far cheaper too. Took a lot longer to do as well. If I think something will take an hr or 2..it usually takes me a day or 2
I would never be a good rigger
the customer would never get the boat It is a bit disappointing the overall quality of the boat and the dealer installed options (or lack thereof). The trailer frame and gelcoat finish is the most disappointing. I do like the disc brakes and the electric lockout for reverse...but I came to accept the Mac for what it is.
More mods have been done and more coming..with lots of pictures to come.
Paul
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NYharleyrider
- Deckhand
- Posts: 34
- Joined: Tue Oct 12, 2004 3:57 am
- Location: Rochester, NY 05 26M, 70 HP TLDI
- Contact:
Hi Mtc... Thanks for bringing this thread back from the past and congratulations on the purchase of your new boat too. My M is scheduled for delivery on April 18 and if all goes well we'll be sailing on that date too.
After much deliberation I finally chose to go with Tohatsu's 70 hp TLDI. Some will tell me I should have gone for the 90 being that they're the same weight, but I'm happy with what I chose.
For me, inspite of the extra HP I think the 70 is a wiser choice for several reasons.
1) No transom modification or strengthening required. Why spend the extra bucks as well as mess with your warranty.
2) While the more expensive 90 TLDI will push the boat faster - from what I've read I don't think it will be significantly faster. This is my unproven theory at this point, but I will post my findings when I have information to share.
3) With the 90 over the 70 TLDI The gas mileage drops by about 25% while the RPM's at wot remain the same.
From Tohatsu's web site, RPM's and Gallons per hour:
90 TLDI: 5500 rpm's at 8.5 gph.
70 TLDI :5500 rpm's at 6.4 gph.
Additionally the 70 can swing the same size prop as a Mercury big foot.
The big foot and the TLDI are pretty much geared the same and I ordered my TLDI with a 14 x 11 prop. Oh and by the way... Tohatsu does make a flush mount control which I ordered with the motor at no additional charge and a trim guage, Tach and prop are also inclduded with the purchase.
With Mercury, Honda and Yamaha you still have to buy the prop and controls as an option.
At the end of the day my purchase price for a brand new left over 2004 70 hp TLDI was only $5050.00 delivered. All in all I think I've got a lot of motor and parts for the money I spent.
After much deliberation I finally chose to go with Tohatsu's 70 hp TLDI. Some will tell me I should have gone for the 90 being that they're the same weight, but I'm happy with what I chose.
For me, inspite of the extra HP I think the 70 is a wiser choice for several reasons.
1) No transom modification or strengthening required. Why spend the extra bucks as well as mess with your warranty.
2) While the more expensive 90 TLDI will push the boat faster - from what I've read I don't think it will be significantly faster. This is my unproven theory at this point, but I will post my findings when I have information to share.
3) With the 90 over the 70 TLDI The gas mileage drops by about 25% while the RPM's at wot remain the same.
From Tohatsu's web site, RPM's and Gallons per hour:
90 TLDI: 5500 rpm's at 8.5 gph.
70 TLDI :5500 rpm's at 6.4 gph.
Additionally the 70 can swing the same size prop as a Mercury big foot.
The big foot and the TLDI are pretty much geared the same and I ordered my TLDI with a 14 x 11 prop. Oh and by the way... Tohatsu does make a flush mount control which I ordered with the motor at no additional charge and a trim guage, Tach and prop are also inclduded with the purchase.
With Mercury, Honda and Yamaha you still have to buy the prop and controls as an option.
At the end of the day my purchase price for a brand new left over 2004 70 hp TLDI was only $5050.00 delivered. All in all I think I've got a lot of motor and parts for the money I spent.
At 5500 rpm, 90HP is 85.9 ft-lbs of torque, while 70HP is only 66.8 ft-lbs of torque. That's why the 90 consumes more fuel at WOT at 5500 rpm than the 70 hp. With more torque, the motor can turn a prop of greater pitch, and as a result, push the boat faster at WOT 5500 than the 70HP can.
Just doing the basic math, the 90 should be about 13% faster than the 70. For example, if the 70 tops out at 20 knots, at 5500 rpm with an 11" prop, the 90 should top out about 22.7 knots at 5500 rpm with a 13-14" prop. This doesn't account for motor weight and prop slip.
At 20 knots, 6.4 gph = 3.125 nmpg and at 22.7 knots, 8.5 gph = 2.67 nmpg. The difference in nmpg is 17% in this example, and that's the price of going faster. If the 90HP is throttled back to where it's only producing 70HP and 20 knots, its mileage will improve and shouldn't be significantly worse than the 70 at the same speed.
--
Moe
Just doing the basic math, the 90 should be about 13% faster than the 70. For example, if the 70 tops out at 20 knots, at 5500 rpm with an 11" prop, the 90 should top out about 22.7 knots at 5500 rpm with a 13-14" prop. This doesn't account for motor weight and prop slip.
At 20 knots, 6.4 gph = 3.125 nmpg and at 22.7 knots, 8.5 gph = 2.67 nmpg. The difference in nmpg is 17% in this example, and that's the price of going faster. If the 90HP is throttled back to where it's only producing 70HP and 20 knots, its mileage will improve and shouldn't be significantly worse than the 70 at the same speed.
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Moe
Follow-up question on Merc 60HP Bigfoot and transom strength
My O/B installer put an aluminum bar between the top anchor bolts.
He stated that this would increase the transom strength since I had chosen to got with a 60HP Bigfoot with the 14x10 prop.
Will this actually strengthen the transom...a one foot long 'backing plate' rather than just the two SS washers at the bolt holes?
Where does the weakness of the Mac transom occur? At the anchor bolt holes or where the transom merges with the stern hull?
He stated that this would increase the transom strength since I had chosen to got with a 60HP Bigfoot with the 14x10 prop.
Will this actually strengthen the transom...a one foot long 'backing plate' rather than just the two SS washers at the bolt holes?
Where does the weakness of the Mac transom occur? At the anchor bolt holes or where the transom merges with the stern hull?
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waternwaves
- Admiral
- Posts: 1499
- Joined: Wed Oct 13, 2004 8:18 pm
- Location: X less in North Puget Sound -have to sail other boats for a while
transom mods
Strengthening the transom....
there are 3 common approaches...
1) a plate stiffer than fiberglass and of sufficient area to spread the flexural load on the fiberglass over a larger surface. Frequently results in cracks immediately around the plate. due to localized stress of metal on glass... Thin polyethyelen plate between the stiffener and the glass (Ala the original BWY 70 hp solution on the M) works nice also..
2) rail extensions, which dont increase the load area, but do move the load closer to the corners of the transom which are stiffer. Larger and uglier.... and also run risk at the fasteners to transom connection for cracking... and not a lot of outboard locations to mount to in the stock X boat anyway.... But seems to be the standard solution for most.
3) internal and or external fiberglass addition to increase transom thickness and and structured channel thickness (box beams and spreaders). Slowest and most difficult to accomplish on the transom and lower hull.. (but the solution I like since no-one can see the reinforcment, and I plan on taking the advertising off the OB cowl anyway....
It is a difficult location to add glass on the inside...., I found fresh air from a powered respirator with a long hose.to be quite nice while working back there.... but next time I think I will go for a full face mask... those fumes are not good for your eyes....
PArts of my transom (where I have drilled through) are thinner than 1/2 inch. There is a lot of deflection of the transom, compared to other boats I own.
I started glass mods for OB, tabs, rudders, and now I want the option for maybe adding a fold down dive deck... This is basicly going to add 23 lbs of resin, beams and fabric to the transom to do this all... (I do want that 90 e-tec...)
And for those that dont want to carry the water in the cb well.....pump it out with air.... the top of the CB trunk is above the water line when the the boat is level....
But really, I am not really seeing a huge problem with that. When I launch from a sling..there is mostly air in the trunk anyway.. now on a sloped ramp I am sure there is more water in it..... but that weight is low in the boat and I am probably not carrying as much water there. and any air in the CB probably releases back into the trunk as the CB is dropped...(less the glass volume of the CB)
WEll, back to measuring for the dive deck mounts.....lol....... I knew I needed a reason to go back outside...... (If I dont finish this duane is going to be back in the water before me.....if he isnt already this weekend....lol)
darren
there are 3 common approaches...
1) a plate stiffer than fiberglass and of sufficient area to spread the flexural load on the fiberglass over a larger surface. Frequently results in cracks immediately around the plate. due to localized stress of metal on glass... Thin polyethyelen plate between the stiffener and the glass (Ala the original BWY 70 hp solution on the M) works nice also..
2) rail extensions, which dont increase the load area, but do move the load closer to the corners of the transom which are stiffer. Larger and uglier.... and also run risk at the fasteners to transom connection for cracking... and not a lot of outboard locations to mount to in the stock X boat anyway.... But seems to be the standard solution for most.
3) internal and or external fiberglass addition to increase transom thickness and and structured channel thickness (box beams and spreaders). Slowest and most difficult to accomplish on the transom and lower hull.. (but the solution I like since no-one can see the reinforcment, and I plan on taking the advertising off the OB cowl anyway....
It is a difficult location to add glass on the inside...., I found fresh air from a powered respirator with a long hose.to be quite nice while working back there.... but next time I think I will go for a full face mask... those fumes are not good for your eyes....
PArts of my transom (where I have drilled through) are thinner than 1/2 inch. There is a lot of deflection of the transom, compared to other boats I own.
I started glass mods for OB, tabs, rudders, and now I want the option for maybe adding a fold down dive deck... This is basicly going to add 23 lbs of resin, beams and fabric to the transom to do this all... (I do want that 90 e-tec...)
And for those that dont want to carry the water in the cb well.....pump it out with air.... the top of the CB trunk is above the water line when the the boat is level....
But really, I am not really seeing a huge problem with that. When I launch from a sling..there is mostly air in the trunk anyway.. now on a sloped ramp I am sure there is more water in it..... but that weight is low in the boat and I am probably not carrying as much water there. and any air in the CB probably releases back into the trunk as the CB is dropped...(less the glass volume of the CB)
WEll, back to measuring for the dive deck mounts.....lol....... I knew I needed a reason to go back outside...... (If I dont finish this duane is going to be back in the water before me.....if he isnt already this weekend....lol)
darren
Go big
My advice:
1. Keep your eye on HP. For me, plowing along at 16mph at a noisy near WOT when fully loaded with a 50 HP really begs for more power. Would be so much nicer to be making 20+ mph at lower and quieter RPMs . I question whether an additional 10 HP would make a significant difference.
2. Start your search by investigating the lightest big engine available that you can get serviced locally. I did this search and concluded the Tohatsu 70/90 fits that need best with the E-tec 75/90 close behind. As I recall the others were much heavier and don't forget that sailboats don't like lots of weight on their stern. I had heard that Evinrude was going to offer a 70HP that weighed in at 235# but the factory advised a local dealer that more development was needed. I eventually bought a used boat with a 50 Tohatsu TLDI. The engine runs fine but for me it is definitely underpowered.
3. In my opinion props are, for the most part, a detail that depends upon your specific usage. There is no prop in the world that will make my 50 do what I want it to. Keep your eye on HP - that's what counts. Then worry about prop size.
1. Keep your eye on HP. For me, plowing along at 16mph at a noisy near WOT when fully loaded with a 50 HP really begs for more power. Would be so much nicer to be making 20+ mph at lower and quieter RPMs . I question whether an additional 10 HP would make a significant difference.
2. Start your search by investigating the lightest big engine available that you can get serviced locally. I did this search and concluded the Tohatsu 70/90 fits that need best with the E-tec 75/90 close behind. As I recall the others were much heavier and don't forget that sailboats don't like lots of weight on their stern. I had heard that Evinrude was going to offer a 70HP that weighed in at 235# but the factory advised a local dealer that more development was needed. I eventually bought a used boat with a 50 Tohatsu TLDI. The engine runs fine but for me it is definitely underpowered.
3. In my opinion props are, for the most part, a detail that depends upon your specific usage. There is no prop in the world that will make my 50 do what I want it to. Keep your eye on HP - that's what counts. Then worry about prop size.