First boat- Small aluminum with motor from a pontoon- Where to start with prop?

[thecadman99]

Hello all,
After getting my first boat (a very old 15' aluminum) I've been told that the motor is too long. Running the boat, I'm seeing a max speed of 19 mph.
The motor is a 1995 Merc 25hp 2 stroke long shaft. The prop is a 10-3/8 X 9.5 pitch.

I haven't had a chance to get a tach yet, and I'm not sure how much my boat weighs (I know I need more info!).

Performance-wise. The boat comes out of the water very quickly and "winds out". I normally run throttled back (afraid of over-revving) and run about 17mph.

Everyone tells me I should be getting much more speed. I checked a much heavier-looking bass tracker boat and it had a 15 pitch prop on it, but I wasn't sure about the diameter.
I usually carry 5 gallons of gas, two batteries, and its just me and the wife (combined about 340lbs)

Here are some shots of the boat. If you guys can point me in the right direction it would be very helpful!

 

[GA_Boater]

Gonna be a little blunt, Cad, You should think about replacing the transom before worrying about more speed. C-clamps holding patch wood onto to what ever is left of the transom, oh my. That isn't very safe. While you're at it, check for wet foam under the floor. Added weight affects boat speed.

BTW - The motor is too low.

 

[thecadman99]

Gonna be a little blunt, Cad, You should think about replacing the transom before worrying about more speed. C-clamps holding patch wood onto to what ever is left of the transom, oh my. That isn't very safe. While you're at it, check for wet foam under the floor. Added weight affects boat speed.

BTW - The motor is too low.

Thanks again for the reply GA_Boater (you helped me out on another thread too!) Good eye on the clamps. The transom is actually in great shape, but the PO had not put aluminum back over the top of the transom on the outer edges, so the very top was starting to separate. I loaded it up with Titebond III and had it clamped in the photos. It turned out very nice and is stiff and wobble-free.

I'm guessing the best way to lower the motor would be to put in a new transom, without the center dip. I don't think I will get to that this year (maybe a winter garage project), but I am really wanting to get the prop closer to optimal. As it is, its just revving too high.

I'm looking at a prop on ebay, but I'm not sure it will fit. Its a 9.25 diameter by 12.5 pitch that came off of a 2006 25hp Mercury 4 stroke.

My prop is a 48 19636 A10 9.5p but I haven't been able to find a place online that can tell me the number of splines on mine (I think its 10).

I'm thinking I may need something as high as a 15p, but again, that is just from reading without any experience with the different pitches and how much 2p would affects rpm on my motor.

 

[Mi duckdown]

99. you have a long shaft motor on a 15 " transom, or less

 

[achris]

Either build the transom up to 20" or change the engine to a 'standard' shaft... The only real way to know if you have the right prop is with a tacho on the engine... At WOT it should be making between 5 and 6 thousand revs....

Chris......

 

[steelespike]

I personally wouldn't buy a prop until you can test drive for wot rpm and gps speed with a 20" transom. An induction tach will work.
You can't tell much about the performance dragging all that motor through the water.
If you insist on a prop I would start with a 12" pitch.With the right setup I think the 25 should make 28-30 mph.
You should get that tach so you know your wot rpm.

 

[Scott Danforth]

You have a longshaft motor on a short shaft transom. either build a new transom or get a short shaft motor. your prop is too deep and the outdrive is plowing, adding drag. you would get much more performance from the proper placement of prop and cav plate with the same hp.

 

[Sea Rider]

If boating with a long shaft sittting on a short shaft transom, at speed will definitely achieve water splashes back transom if water flow (WF) passes betweeen AV plate and small upper water deflector plate and over transom if WF pases above small water deflector plate, the one located next to swivel bracket. Both will slow you down and combo will perform satisfactorilly at just slow/fast displacement speeds.

Go for a wot spin with lightly loaded boat and visually have a mate check at back transom what's going on so to determine at which tail height the splash is being produced, then will know what to do if going for a transom mount, raising transom accordingly, etc. Any of both resources will be cheaper than going for a short shaft engine.

Once at the Sweet Transom Height install an induction (Hardline/TinyTach) and check final wot rpm, latter can play with + prop pitch to be near max wot rpm factory stated for that engine and boat happily forever.

Happy Boating

 

[thecadman99]

Thanks for the helpful replies everyone.

Would you all recommend replacing the cut-out transom with a straight one llike the boat on the left here:

Or using some aluminum channel and laminated ply or a treated 2X6 like this:

 

[Sea Rider]

If your actual transom has a middle cut as in right pic, before modifying or adding any device go for a wot spin to determine ideal transom height, then could make transom straight as in second pic, if in actual state you achieve severe water splashes out/over transom, when rising transom to be even, will achieve a much better lower leg performance than before.

This is not a rocket science issue, lots of variables to deal with, but you need to start somewhere. Wot spin is mandatory, report back your findings. Engine must sit on transom perpendicular to water level, dial best trim hole to achieve this cond.

Happy Boating

 

[thecadman99]

If your actual transom has a middle cut as in right pic, before modifying or adding any device go for a wot spin to determine ideal transom height, then could make transom straight as in second pic, if in actual state you achieve severe water splashes out/over transom, when rising transom to be even, will achieve a much better lower leg performance than before.

This is not a rocket science issue, lots of variables to deal with, but you need to start somewhere. Wot spin is mandatory, report back your findings. Engine must sit on transom perpendicular to water level, dial best trim hole to achieve this cond.

Happy Boating

I've had the boat out, and tried every trim pin hole (settling on the second highest) which gave the same speed as the highest pin setting. I couldn't plane as well and noticed about 2mph less on the lowest pin setting. \

I think I have a non-contact tach (if I can find it in all my junk) but I did note that at WOT on plane the motor was running at such a high rpm that I throttled it back. It came on plane in about 8 seconds.

I had no water at all in the bottom of the boat after several runs with a passenger, and didn't notice any water washing up between the motor and boat, but I will check at my next opportunity to take a run.

Thanks again for taking time to offer advice.

 

[Sea Rider]

Unless you have a trained ear tuned to hear 5-6 K rpm at wot, the consensus is that will need to install an induction tach to know max achieved wot rpm. When boat is on plane, prop shaft, Av plate must ride parallel along boat to water level, so it's useless to play with trim angle and vary this spot on settting.





If you vary trim angle will lose best prop thrust and overall speed as you have experienced, prop must bite water flow perpendicular to it, specially if running boats with portable engines in which you need to get the max punch out of them. Is it possible to bump that 25 into a 30 HP ? 20% more will surely make the difference.

Happy Boating

 

[steelespike]

In the final picture of motor it appears it is about 5" low but a nice smooth section of the leg is at the water surface and there is still one remaining deflector
to help control splash.Just make yourself a nice jack plate to get the motor up Then you can fine tune the height for least drag and yet able to resist venting.