Sailboat propeller question
 

Dennis wrote:

Hi Roger and everyone who replied!

Thanks so much for the constructive feedback!

Couple of points to clarify:
- I forgot to mention, the gear ratio is about 1:2.06 (can't read the

I think your gear ratio should be closer to at least 3.0. Your transmission

sounds like it is configured for a planing power boat.

Sherwin D.


label on the gearbox)
- I recently had the bottom done completely - it is as smooth as it
gets.
- I also had the propeller checked by the manufacturer and they
confirmed that the pitch is what it has been manufactured at.
- I have a brand new shaft and coupler and these are properly
connected.

So, I guess my best bet is to check the transmission. Now, how do I do
that?
- Can I check the clutch/transmission without disassembling everything?
Any ideas?
- Can I check and repair the transmission without hauling the boat out
of the water?
- I also notice that there is a very high pitch noise from the engine
after about 3700rpm until the max. rpm. Could that also indicate a
toasted transmission?

Thanks again!
Dennis

Sailboat propeller question
 

"sherwindu" wrote

I think your gear ratio should be closer to at least 3.0. Your
transmission

sounds like it is configured for a planing power boat.

No, that's a fairly typical reduction for a sailboat. The greater the
reduction, the larger the prop. Sailboats usually compromise
effeciency under power with smaller reduction ratios and props in
order to reduce drag under sail.

--

Roger Long

Sailboat propeller question
 

28 foot boat so waterline length about 23 feet.

7800 pounds displacement, probably from brochure, say 8200 really or
2.31 long tons.

10.5 horsepower, probably 9.5 after charging batteries and with
transmission and shaft losses.

With the right prop, you should easily be getting 6.5 knots.

3500 rpm (I can't imagine you want to run that fast) / 2.06 = 1699
shaft RPM

Va say .87 x 6.5 knots = 5.66 knots so a skinny two blade prop should
about 10 inches in diameter. You are right on the money.

A 10 inch prop with these numbers should have a pitch to diameter
ratio of 1.15 pr 11.5 inch pitch so you are a bit light on pitch.
Efficiency would be about .36 with the right wheel.

Your numbers (including the lightly pitched wheel) are all pretty
typical for a sailboat. There is something else going on here.

--

Roger Long



"Dennis" wrote in message
oups.com...
Hi!

I've got a 28 foot sailboat, 7800lbs with a 10.5hp engine. It has a
3
blade prop with 10 inch diameter and 9.3 pitch. Prop is in like new
condition, so is the bottom. When I sail, I can push her up to 6
knots.

Though, when I spin the engine at 3600 rpm I only get about 3.5
knots
out of the boat. The engine easily manages more than 4000 rpm, so I
don't think it's an engine problem. I suspect that the prop has the
wrong pitch, but according to the manufacturer it's exactely the
right
one.

Why don't I get more than 3.5 knots under power? Does anyone have a
similar configuration or any clues?

Thanks!

Dennis

Sailboat propeller question
 

The analogy with an outboard may not carry over to an inboard, but on my
sailboat
there is a distinctive improvement in performance using a 'pusher' type engine
gearing. Most outboards that do specify their use on sailboats usually have
gear
ratios 3.0 or higher. The best pusher I ever used was my British Seagull 6
bladed
prop with a gear ratio of about 4:1, and I saw pictures of this engine pushing
huge
heavy barges.

Roger Long wrote:

"sherwindu" wrote

I think your gear ratio should be closer to at least 3.0. Your
transmission

sounds like it is configured for a planing power boat.

No, that's a fairly typical reduction for a sailboat. The greater the
reduction, the larger the prop. Sailboats usually compromise
effeciency under power with smaller reduction ratios and props in
order to reduce drag under sail.

--

Roger Long