Evinrude 15 not very fast

I have a 1975 evinrude 15hp on a 14ft semi v. Very lite boat. I had myself and 2 other people in it and it would only go 7.5mph. With just myself in it it would go 15mph. The prop is a 9 1/2 x 10. Is that all it has?

Probably too much of a load with that prop, which most likely was a light load or 'speed' prop.

Try this to see what it would recommend for that loading: https://forum.tinboats.net/viewtopic.php?f=4&t=37433

If you can't find your exact hull, find a known brand (Lund, DuraNautic, Sea Nymph, etc.) and use that.

Is the boat factory light or have you "modded" it with platforms, etc.?
Would think with proper engine trim and a properly performing engine that the boat should run lower to mid twenties with that power, bare minimum.
Should plane off with two pretty easily as well.

Factory light just three factory aluminum seats

Could me having a hydro foil on it be hurting?

Unless you absolutely need the foil to plane off the boat or stop porpoising I would remove it. Lots of parasitic drag.

I have a 15 hp Yamaha on a 1648 aluminum flat boat. Plywood floorboards. With two adults we were going 17.5 mph according to the gps. I have no idea what kind of prop is on it though. I would think that it would push your 14 better than that

Boosted97gt said:

I have a 1975 evinrude 15hp on a 14ft semi v. Very lite boat. I had myself and 2 other people in it and it would only go 7.5mph. With just myself in it it would go 15mph. The prop is a 9 1/2 x 10. Is that all it has?

Click to expand...

I've got a 78 Evinrude 15 running a 9.5x8 prop it will push the 14'' approx 19 to 20 mph . On the flip side the mid 80's 9.9 which I really think is a 15 will push the same boat with the same load 24 mph ,it's running a 9.5 x 10 prop. What I'm getting at is If your points ,plugs , compression are good , I really think your over propped. Those years the power was rated at the power head, in the mid 80's it was rated at the prop.
Like what Pap stated if ya can get rid of the fin it will help your top end. Hope this helps

maybe someone took the carb off of it and replaced it with a 9.9

you have a lot of prop on that v hull ... try a 8 or 9 pitch .... should give you way more lift and even more top speed .... my 1648 hadda 9 pitch on a 9.9 and loaded it wouldn't get on plane and the rpm's were way low .. we propped it out with a 9x7 4 blade for those heavy loads ... and its faster with the 7 than it was with the 9 .. and it jumps on plane FAST.

I have a 74 15hp Johnson on my 16' semi-vee Ouachita. It will run 17-18 mph (GPS) with two 220+ people and trolling motor, battery, gear, etc. 9.5x10 pitch prop with one slightly misaligned blade (3-blade standard prop).

I've got basically the same setup, 14ft Alumacraft, 80 model Evinrude 15, same prop, and I run solo rigged (battery, trolling motor, fishing tackle), about 23mph.
Have you tried a tach to see what rpm you are running?

Finally got it out with a tach. Just me in it it was 4800rpm with me and one other guy (both of us 200lb, two Rods a tackle box and 6gallon has tank) it was 4900rpm.

OK

What's the engine rated for WOT RPM?

4800 seems really low.

Full power operating RPM for the 9.9 is 4500 - 5500. The 15 hp motor is rated at 5500 -6500 RPM.

What is your model number?


You may have a 15hp hood on a 9.9hp motor or a 9.9hp carburetor on a 15hp model motor.


The below information was obtained form Leorys Rambling site.
The outside visual dimensions of both carburetors appear the same, unless you look INSIDE the throat. If you look in from the rear, you will have to position the throttle plate to a horizontal position to be able to see the throat very well. The 9.9 hp carburetor’s INTERNAL THROAT is .625 dia., 15 hp internal throat is .875 dia. up until late 1987 when they went to a plastic topped carburetor. NOTE this is not measured from either the from or back but the internal throat that you can see on the LH photo below. The outlet or rear throat dia. is the same on both to match the manifold port dia. The idle jet venturi system is also slightly different between the two. The part number for the the pre 1987 9.9 hp Carburetor Assembly is #388936, while the 15 hp Carburetor Assembly is #338273. This increased throat dia. and larger main jet allowed it to breather better and to pull more fuel in, increasing the RPM about 1000 RPM, hence more HP.

Model 15504c.

Measure the internal venture and compare the diameter to measurement posted on the
previous post.

Make sure throttle is wide open. years ago we got a new to us 25hp, 1st few times out I thought it was slower than our old one, found out the twist grip went to a point then got harder to turn. Going past that was wide open, and fast!