Help please with 4hp propellor issue

[Jack1991]

Hey everyone, I was just wondering whether anybody can help me out with a prop question. I have an old-ish mariner 4hp on a small tinny and the prop on it is a standard 3 blade 7.5 x 7-B

When i get to about half throttle acceleration stops but you can tell by the sound of the motor and throttle position that there should be a little more poke there...
Should i increase/decrease pitch and or diameter because my friend has a similar prop on his 4hp that has longer/skinnier blades and he said to try fet one like that??

Any help would be greatly appreciated!!

[gofishin]

Jack, you probably won't have that many prop options, in terms of diameter, at that end of the scale.

From what you describe you need a smaller pitch, however, just keep in mind that it is only 4HP. What boat type/size, and how many people?

First thing I would do is try your mates prop on your motor & boat. Also compare the numbers on his prop to yours.
Cheers
Brendon


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[Jack1991]

Well that's another problem because my prop is the shear pin style and his has the spline even though there wouldn't be 10 years difference between them..

I had a 3.4m V hull stacer "lightweight" and I've recently downsized to a 2.5m flat bottom punt.
Generally it is only myself and either one child or one adult. Even when I go by myself in the 3.4m and the 2.5m either sitting back or middle it still feels held back (rpm wise).

I was also told the bottom of my boat should be in line with the plate that covers the top of the prop (cavitation plate?), for optimum speed so I've tried experimenting with lifting the outboard an inch or so and also adjust the actual angle from straight up and down to nearly sticking it out the water to no avail.

Thanks again!

[gofishin]

... I was also told the bottom of my boat should be in line with the plate that covers the top of the prop (cavitation plate?), for optimum speed ... Yep, not above the bottom, sometimes a bit lower to allow for interference from the keel extrusion. The 3.4 will plane with that motor with one adult, sitting on the middle or fwd seat/thwart if there are only 2 seats (tiller extension reqd). The 2.5 punt will be harder to plane, even though it's lighter. Short fat boats don't plane easily.

However, either boat will never be a 'speed machine' and in reality will be a displacement boat with how you are using it. 4HP is the limiting factor.


... and also adjust the actual angle from straight up and down to nearly sticking it out the water to no avail... straight up & down, even slightly in toward the transom. Nearly sticking out will be useless.


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[aussiebasser]

You need a prop with more pitch if you are not getting more speed at full revs. Problem is you have a 4 hp motor. If you go to a higher pitch, the motor probably won't push the hull out at low revs. Bit of a catch 22 with a 4hp which is why manufacturers put a compromise prop on it. If you wanted to go the whole hog, you could send your prop out to be cupped, which will give you benefits at higher revs, however again, with a 4hp, this probably won't work either.

[Gon Fishun]

Are you sure it is the prop and not a fuel/elec/tuning issue.