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

This probably has as much to do with propeller blade area as pitch.
You post omits one vital piece of information which is the reduction
gear ratio. You should be able to find this on the data plate affixed
to the side of the transmission.

Another important piece of information to determine a fix is the
distance from the shaft at the center of the propeller to the hull or
the current prop tip to the hull.

The type of engine would be useful.

--

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

Is that 4000 RPM under load or is it in neutral?
Dave

Roger,

I think you are on the right track. Sailboat hulls are deep displacement
hulls
that need lots of 'pushing' power. The usual problem in this case is that the
engine
is not geared high enough to develop the pushing power required. Spinning the
prop
at high rpm is just going to produce a lot of turbulence. You need lots of
torque
to develop at lower rpm of the engine. Another words the engine has to
be geared higher to produce less revolutions of the propellor for any given
engine
speed. The best example I know of is the old British Seagull outboard with a
gear
ratio of about 4.0 which was capable of pushing huge barges, albeit slowly.

Sherwin D.

Roger Long wrote:

This probably has as much to do with propeller blade area as pitch.
You post omits one vital piece of information which is the reduction
gear ratio. You should be able to find this on the data plate affixed
to the side of the transmission.

Another important piece of information to determine a fix is the
distance from the shaft at the center of the propeller to the hull or
the current prop tip to the hull.

The type of engine would be useful.

--

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

On 18 Oct 2006 20:33:09 -0700, "Dennis"
wrote:

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

I ran a few numbers, but numbers take you only so far....

If, I say IF you were putting 10.5 HP into thrust at 100% efficiency
and making 3.5 kts, then the hull would be opposing the engine with
975 lb of drag. If you can find someone to pull you along at 3.5 kt
with a spring scale on the line, you could find the actual drag (which
is certain to be much much lower)

From the other end of things: If, I say IF the prop were developing
thrust at 100% efficiency and no slip, 3600 RPM and 9.3 inch pitch
would provide 27 kts water flow immediately behind the prop and maybe
13 kt over the water.

I conclude your numbers don't hang together well.

Brian Whatcott Altus OK

Propeller efficiency would be at best .53 and probably as low as .33
for and auxiliary sailing vessel.

--

Roger Long



"Brian Whatcott" wrote in message
...
On 18 Oct 2006 20:33:09 -0700, "Dennis"
wrote:

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

I ran a few numbers, but numbers take you only so far....

If, I say IF you were putting 10.5 HP into thrust at 100% efficiency
and making 3.5 kts, then the hull would be opposing the engine with
975 lb of drag. If you can find someone to pull you along at 3.5
kt
with a spring scale on the line, you could find the actual drag
(which
is certain to be much much lower)

From the other end of things: If, I say IF the prop were
developing
thrust at 100% efficiency and no slip, 3600 RPM and 9.3 inch pitch
would provide 27 kts water flow immediately behind the prop and
maybe
13 kt over the water.

I conclude your numbers don't hang together well.

Brian Whatcott Altus OK

Dennis, I had the same problem on my Tanzer 26 and Yanmar 1GM a while.
Found out that the injection pump regulator screw has free up itself, so
even if the engine was able to turn 4000 rpm unloaded, it was not a 7HP
anymore but barely 1-2 HP.

Do not discard the engine as the cause yet. I would rather find the reason
in the engine than elsewhere since your hull is clean.

"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

Hi André!

When I mentioned that the engine revs up to 4000rpm, that is under load
(i.e. in forward or reverse gear). I presume if the engine only had 1-2
HP, this would not be possible?

Thanks a lot!

Dennis

Dennis wrote:
Hi André!

When I mentioned that the engine revs up to 4000rpm, that is under load
(i.e. in forward or reverse gear). I presume if the engine only had 1-2
HP, this would not be possible?

Thanks a lot!

Dennis

My 28' S2 weighs 8000 lbs and came with a Yanmar 1GM with 6.5 hp. With
a 3 blade prop. I could get 5 kts in calm water with clean hull and
prop. I did not like this because I was always afraid that in a
thunderstorm situation that I would not have enough power. I also
wanted a bigger engine cuz I thought I would do a lot of motoring whn
cruising so I got a 13 hp 2GM. With the 2GM, I can easily get to
nearly 7 kts under power.

There is an identical S2 next to mine equiped with the 6.5 hp 1GM.
The owner says he has the 3 blade prop and last week he told me he only
gets 3.5 kts out of it. Something is wrong here and I dont know what
but i seem to have lucked out.

A sailing prop or a motoring prop? The best of both worlds? Variable /
folding prop? You need a low speed prop, but not too fine. One sheet of
luan and 3.5 hp will go 30 knots, but develops best thrust going fast,
coarse pitched. You want high rpm, going slow. Fat blades are best, but
drag most when sailing. 3 blades sounds like a tugboat, with torque but
no speed. Could it be too fine?

My two bladed prop is 13R12

10.5 Hp two cylinder sounds like a universal medallion atomic 2. 12 HP?

I've got one. It has a WICO dog tooth magneto on a gear train out the
left looking fwd side, and that also drives the water pump.

A head gasket was an early problem. My head seldom gets hot, barely
warm, no thermostat. So long as the impeller is OK, that is.

Floods out when hot and power reduced. Work it all day, turn it off
without a cool down and pop goes your head gasket, first delivery.

I have no knotmeter, no tach. My timing is accomplished by rotating the
body of the magneto, having loosened off it's nuts while under power.
The one below is loosened before starting, the one above, under power,
or idling.

There is no spark advance to accommodate high rpms, but a manual lever
seems possible. This is more basic than Ford's "T", though it does have
a front mounted electric starter and flywheel.

I have about 100 pounds dry compression, each cylinder. An old Zenith
carb with leaky top and bottom shaft seals makes getting the mixture
right a pain. I have wired rubber grommets around the shaft bearings,
things are better, now. I think the idle jet is plugged.

Normally, at idle, I leave 1" of wake, at wot, about 6". Pleny of
power. I could look up the reduction ratio.

I have a 2 bladed prop, with a small blade set, 13R12. I have a low
drag asymmetrical bilge keeler, Tyler 29 6800 lbs. I don't bother with
a shaft lock, but could turn the flywheel, and mark the shaft for a
vertical propellor, half hidden behind the skeg. A mirror or webcam
might be helpful, there. The single exposed blade would make the boat
turn right under sail.

I sail in a river. I have a larger prop, steeper, but never tried it.
Once under sail, I don't care how fast I'm going, really.

Westerbeke carries spares and sells xeroxes of parts lists, etc.
Ridiculously expensive. Half shell bearings 65 bucks per half. I
ground down some 3 dollar shells to fit it. I'll Make rings out of
fence wire or a slinky, some day. I'll have to buy an anvil.

I've never seen the cam shaft or valves, except for when I replaced the
head gasket, but the adjustment is easy, so long as the engine is
upside down, or carb, water pump, magneto, timing gears and covers and
flywheel are removed. The valve access panel leaks oil something awful.
Perhaps the new gasket there has solved that problem. I think an
exhaust valve stuck open for a minute, once, about 5 years ago. The
exhaust header got white hot until a different idle speed was
commanded. It sounded funny.

The oil pump looses a ball bearing, and it's function, when removed
with the bolts extracted from the pump body. This ball bearing can be
retrieved from within the front main bearing cage by shaking the block
while right side up. Do it on a clean sheet of something, it's hard to
pick out a small ball bearing from sawdust and spider web dust bunnies.
The engine will run with this extra ball loose in the crank bearing,
but will develop no oil pressure. Don't ask how I know.

I no longer suffer from fuel starvation caused by a too warm fuel line
and vapor lock, but my pancreas has settled down, and now I need to do
crunches, lots of crunches. After belly muscle repair surgurey,
likely.

If more info might help, let me know. I'll dig out the book. Got a
picture of your engine? If so, send it to me, not the group, or post it
and send a link.

I do have a nice self tacking jib.

Terry K