Boat speed under power....

[Chris]

....As a former CD owner (now a Bristol 24 owner), I wanted to pose this question on your board, as there are many extremely knowledgable sailors here.

The Bristol 24 I purchased has a perfectly running 9.9 Johnson sailmaster (long shaft) with a 10 x 7 propellor (standard with the motor). I calculated my theoretical top hull speed at ~5.7 knots based on 18'1" LWL. I also used a formula that calculated theoretical max. horsepower needed to push my boat to hull speed, based on a figure of 6400 Lbs. displacement, as ~9.3 HP. My problem is that I can only get the boat up to 3 knots max. under full power. I actually have no way of determining WOT (full throttle RPMs) as there is no tach. hooked up., but I'm wondering if it's the propellor that needs to be changed (larger dia., etc.) to reduce slippage - if that's what's even the cause.

The engine sits level in the water & is properly submerged so I don't think it's a ventilation problem. The setup is similar to the CD25/26 with the lazarette locker that houses the engine -- so the lower unit never tilts up. Although there is some growth on the motor's lower unit, anti-fouling has been used & it's not extensive.

Has anybody else had this type of problem on their 25/26s? Any suggestions or possible solutions?

Thanks!
Chris



chill2772@yahoo.com

[Tom G]

Is the 3 knots top speed on a windless day with a slack current, and a clean bottom?

....As a former CD owner (now a Bristol 24 owner), I wanted to pose this question on your board, as there are many extremely knowledgable sailors here.

The Bristol 24 I purchased has a perfectly running 9.9 Johnson sailmaster (long shaft) with a 10 x 7 propellor (standard with the motor). I calculated my theoretical top hull speed at ~5.7 knots based on 18'1" LWL. I also used a formula that calculated theoretical max. horsepower needed to push my boat to hull speed, based on a figure of 6400 Lbs. displacement, as ~9.3 HP. My problem is that I can only get the boat up to 3 knots max. under full power. I actually have no way of determining WOT (full throttle RPMs) as there is no tach. hooked up., but I'm wondering if it's the propellor that needs to be changed (larger dia., etc.) to reduce slippage - if that's what's even the cause.

The engine sits level in the water & is properly submerged so I don't think it's a ventilation problem. The setup is similar to the CD25/26 with the lazarette locker that houses the engine -- so the lower unit never tilts up. Although there is some growth on the motor's lower unit, anti-fouling has been used & it's not extensive.

Has anybody else had this type of problem on their 25/26s? Any suggestions or possible solutions?

Thanks!
Chris

tgrant9008@aol.com

[Chris]

Tom,

Yes, 3 knots is max. on a windless day. The bottom is reasonably clean - as clean as can be expected after being in the water since May.

[Eric]

Just guesses but I'd suspect either the prop is slipping somehow (should be easy to check, put it in gear with the engine off and try to turn it), or you have a bed of musles or something growing on the bottom (maybe way down)

....As a former CD owner (now a Bristol 24 owner), I wanted to pose this question on your board, as there are many extremely knowledgable sailors here.

The Bristol 24 I purchased has a perfectly running 9.9 Johnson sailmaster (long shaft) with a 10 x 7 propellor (standard with the motor). I calculated my theoretical top hull speed at ~5.7 knots based on 18'1" LWL. I also used a formula that calculated theoretical max. horsepower needed to push my boat to hull speed, based on a figure of 6400 Lbs. displacement, as ~9.3 HP. My problem is that I can only get the boat up to 3 knots max. under full power. I actually have no way of determining WOT (full throttle RPMs) as there is no tach. hooked up., but I'm wondering if it's the propellor that needs to be changed (larger dia., etc.) to reduce slippage - if that's what's even the cause.

The engine sits level in the water & is properly submerged so I don't think it's a ventilation problem. The setup is similar to the CD25/26 with the lazarette locker that houses the engine -- so the lower unit never tilts up. Although there is some growth on the motor's lower unit, anti-fouling has been used & it's not extensive.

Has anybody else had this type of problem on their 25/26s? Any suggestions or possible solutions?

Thanks!
Chris

[Gary L.]

Chris,

There are many factors at work here. Sometimes the horsepower of the engine is less than what is transferred to the lower end of the shaft.
Also, any fouling of the prop could reduce the speed. I recently cleaned the prop of my CD30 and found that it doubled my boat speed at less rpms; and my prop wasn't too badly fouled. Fuel also may affect the speed. Are you using clean fuel?

As a previous owner of a CD26 with a 9.9hp Yamaha 4 stroke, I found that on calm water, no current, no wind, I could do the hull speed.

Also, are you using a knot meter? This could be off. Test it against a known distance (round trip to get an average speed)


Good luck,

Gary Lapine
Red Witch III
CD30C, #339
Somerset, MA



dory26@attbi.com

[Will. W.]

It sounds like there is a problem with the motor. Check for fouled plugs first. Stuff in the carb. could affect performance this way.
Even if you had the wrong propeller (which seems unlikely on a sailmaster), a 9.9 hp should still be more motor than you need. You should be getting to or near hull speed with only 3/4 throttle. I have a 6hp Evinrude thats getting on in years and it pushes me at just over 5 knots at full throttle.

Will Wheatley
Suzi Q
CD25
Sailing From Chesapeake Beach, MD

....As a former CD owner (now a Bristol 24 owner), I wanted to pose this question on your board, as there are many extremely knowledgable sailors here.

The Bristol 24 I purchased has a perfectly running 9.9 Johnson sailmaster (long shaft) with a 10 x 7 propellor (standard with the motor). I calculated my theoretical top hull speed at ~5.7 knots based on 18'1" LWL. I also used a formula that calculated theoretical max. horsepower needed to push my boat to hull speed, based on a figure of 6400 Lbs. displacement, as ~9.3 HP. My problem is that I can only get the boat up to 3 knots max. under full power. I actually have no way of determining WOT (full throttle RPMs) as there is no tach. hooked up., but I'm wondering if it's the propellor that needs to be changed (larger dia., etc.) to reduce slippage - if that's what's even the cause.

The engine sits level in the water & is properly submerged so I don't think it's a ventilation problem. The setup is similar to the CD25/26 with the lazarette locker that houses the engine -- so the lower unit never tilts up. Although there is some growth on the motor's lower unit, anti-fouling has been used & it's not extensive.

Has anybody else had this type of problem on their 25/26s? Any suggestions or possible solutions?

Thanks!
Chris

willwheatley@starpower.net

[tom G]

Tom,

Since you say the engine is running perfectly. I would investigate if the long shaft is putting the prop too far down in the water. Do you have pushing power but no speed ? If so it's worth checking before spending money on a new prop.

Tom
Ambuscade

Yes, 3 knots is max. on a windless day. The bottom is reasonably clean - as clean as can be expected after being in the water since May.



tgrant9008@aol.com

[Gary L.]

Tom,

Yes, 3 knots is max. on a windless day. The bottom is reasonably clean - as clean as can be expected after being in the water since May.

Chris,

Two questions: 1) Has this always been the case? and 2) If not, do you use the outboard everytime you sail?

When I had my CD26, I had to have the lower end of my outboard repaired. I simply did not use the engine enough, I sailed to and from my mooring, which caused a problem with seals, etc. While the engine itself ran perfectly, the lower end was not. I later motor sailed regularly to keep it operating.

Hope this helps,

Gary Lapine
Red Witch III
CD30C, #339
Somerset, MA

[Dave]

I had this problem this year. In the begging of the summer I could push my CD25 at about 5.6 knots. As growth starts the speed eventually gets down to about the high four range. But, last time I cleaned the bottom I didn't clean the engine because the boat was roling back and forth and didn't want to incure an ingury. After that cleaning the speed actually dropped down into the three range about two weeks after the cleaning. I went under again and found a ton of muscles and growth on the lower unit. I couldn't really see it that well from above, but after I cleaned it all off I could make about hull speed. So, I would say that you have some good amount of growth down there and it should be fine after a good cleaning. I wouldn't get into alot of engine diagnostic testing or anything until you make sure that it's not an easy fix.

Dave
CD25 R&R
Gloucester Ma

....As a former CD owner (now a Bristol 24 owner), I wanted to pose this question on your board, as there are many extremely knowledgable sailors here.

The Bristol 24 I purchased has a perfectly running 9.9 Johnson sailmaster (long shaft) with a 10 x 7 propellor (standard with the motor). I calculated my theoretical top hull speed at ~5.7 knots based on 18'1" LWL. I also used a formula that calculated theoretical max. horsepower needed to push my boat to hull speed, based on a figure of 6400 Lbs. displacement, as ~9.3 HP. My problem is that I can only get the boat up to 3 knots max. under full power. I actually have no way of determining WOT (full throttle RPMs) as there is no tach. hooked up., but I'm wondering if it's the propellor that needs to be changed (larger dia., etc.) to reduce slippage - if that's what's even the cause.

The engine sits level in the water & is properly submerged so I don't think it's a ventilation problem. The setup is similar to the CD25/26 with the lazarette locker that houses the engine -- so the lower unit never tilts up. Although there is some growth on the motor's lower unit, anti-fouling has been used & it's not extensive.

Has anybody else had this type of problem on their 25/26s? Any suggestions or possible solutions?

Thanks!
Chris

ddsailor25@ureach.com