You cannot change the laws of physics....

[northwind]

I have a 8hp sailmate outboard on my super seal. (Displacement about 5000lbs).

Currently it has a high thrust prop fitted, however we struggle to get 4knots out of the engine, on calm water.

Do the panel think if I fitted a standard prop this would give us a performance boost?

I should add the boat has a clean bottom.

 

[Rigger Mortice]

I doubt it but, if you put a tacho on the engine as it is, you can see what it is revving at when you are on full throttle. If it is getting up to full revs as it is a new prop probably won't improve things.

You can get tachos/hour meters for under a tenner on E-Bay which is cheaper then getting a new prop that may or may not be any good.

To be honest your engine isn't over powered for a 26 foot boat.

Are sure your log is calibrated properly?

 

[Stemar]

Something doesn't add up there. My Tohatsu 3.5 can push a 24ft Snapdragon at 4kt in flat water with no problem and she weighs at least as much. (3T when I was craned back in this year - must unload some of the kit I'm accumulating!) though as RM says, 8HP isn't exactly overpowered.

Could you borrow another motor from somewhere to compare? If not, +1 for the rev counter as a first step. They're also handy for letting you know she needs a scrub as you find you need more revs for a given speed than you used to.

 

[seumask]

We had a similar boat, a Hunter Delta, that was also outboard powered by a 8 hp outboard engine with a High thrust prop and then a 6 hp with a standard prop. With the standard prop we did not notice any significant difference in forward performance, but we did notice a little difference in using reverse to stop which TBH did not matter significantly. I always thought that you would get better forward performance out of a standard prop with a reasonably slippery hull like the Seal, I have no sicentific reason and if you read this forum many swear by the "High" Thrust prop, however the dealer we bouth the 6 from said that in his experience people did not often come back for a high thrust prop when they had a standard prop on a sail boat and they were perfectly adequate in his opinion. We expected to cruise at 5 Knots in smooth water however this was significantly slower if there was much chop and or a head wind, which meant that it was always better to sail when there was any useful wind!

 

[reeac]

Calculating from average fuel consumption of 3 to 3.5 hours / gallon and assuming 20 bhp.hours / gallon my Sadler 29 with VP2002 usually takes 6 bhp from the engine and that's for a 29 ft., 8300 lb. boat. Suggests that you have a prop. mismatch there.

 

[William_H]

A high thrust prop is equivalent to using a lower gear in your car. It is possible that the engine is at full revs at 4 knots.
hence yes a standard prop could give you more speed at lower engine revs so less noise. (as said check with a tacho)
However the high thrust prop comes into its own if you are bashing into big head wind and waves. (It will maintain your 4 knots because it has more thrust like torque) where you standard prop might load the engine so much under strong head wind that you can not make 4 knots let alone the 5 you can get in calm conditions.
So you must decide if you want the safety of thrust in bad conditions and lower top speed or if you want higher top speed at the risk of poorer performance in a big headwind. NB tide flow does not come into it just wind and wave.
That is the theory. You may just have a bad engine that you can't do better than 4 knots. (through the water of course not GPS speed.)
In my case a smaller boat I am happy with standard prop and willing to try to sail when the wind comes up you may not. good luck olewill

 

[jwilson]

As others have said, 8hp ought to give more than 4 knots on a fast hull such as a Super Seal. I've sailed similar weight boats in the past with 4 hp and that was about the speed we got. Is the engine revving hard? Could it be as simple as throttle cable settings not giving full power?