Motor size for boat

[droolbot]

Looking at buying a 1968 15 ft aluminum Starcraft runabout. Boat is in great shape, but needs a motor. Not looking to move real fast, so is there ANY reason that a 15 hp 4 stroke would not work? It is rated for 50 hp. Would running that low of hp motor put stress on motor if going above trolling speed? Thank you for any help.

 

[rallyart]

I guess the quick answer to your question is, yes, as the motor will be running at 100% throttle at all speeds above trolling. It will probably never get on plane so at any speed above about 4 1/2 MPH you will just be pushing water and nor gaining any speed. It would not act as a planing hull. You might get away with 30hp but I don't know that hull. I would put on a 40hp if I was looking at keeping the price down a bit. You don't see people complaining about over powered boats they own. You often do about underpowered ones.

If you are happy with a 4 MPH boat then you are golden, but then you could just buy an 8

 

[ahicks]

SO much of this call is going to depend on what you are expecting this boat to do. If you just want to plop it into a small lake and do some fishing, I think the 15 would be great. If however, your favorite fishing hole is 5 miles from the only boat landing, that's going to be quite a ride in a boat that may not even plane out.

4 strokes make awesome trolling motors, especially the smaller ones this boat would use. The downside is that they can be heavy, so you need to pay attention regarding anything bigger than say a 20 hp 4 stroke. Compare the 4 stroke weight that you are considering to the 2 strokes which were the only thing around when that boat was rated. E.G. pretty sure a 40-50hp 4 stroke is going to be quite a bit heavier than a 40-50 hp 2 stroke.

 

[droolbot]

I agree on the weight. Tried this motor as a tiller on a smaller boat. Me and the motor on the back was not a good combination. The newer boat is a runabout, so the weight distribution will be much better. Will not need to get anywhere fast, but was not sure if this would overwork the motor while getting from points A to B.

 

[Sea Rider]

What's the total weight of the boat, state max number of passengers rated for. With how many do you intend to go boating with with a 15 HP motor ? The only possible stress for that motor will be powering it at full throttle as loaded while the motor runs towards the min or much less wot rpm range factory stated which is real bad.

Bear in mind that 4 strokes compared to same 2 strokes models doesn't respond as fast, but a correct prop maximization will do the trick to power that combo more nicely. Have you purchased that 4 stroke 15 HP motor or depends on the answers given here to go that route ?

Happy Boating

 

[droolbot]

Already have it. The Starcraft I am purchasing will be without motor.

 

[roffey]

My 2 cents, try it. You have nothing to lose. If its as bad as predicted then look at repowering and keep the 15hp as a kicker. I have seen lots of 30-60 hp motors up for grabs.

 

[Scott06]

My 2 cents, try it. You have nothing to lose. If its as bad as predicted then look at repowering and keep the 15hp as a kicker. I have seen lots of 30-60 hp motors up for grabs.

Exactly if you have it use it, but I have yet to own a boat with too much power, generally max hp is right where you want to be. You can always upgrade down the road. Just make sure if you buy with out engine you pay a low enough price that you don’t end up spending more than you would have now buying a boat with engine.

 

[JimS123]

When I was a widdle kiddy we had a 14 foot woodie that was probably heavier than your tinny. She was equipped with a 3.5 HP Wizard. Obviously, when we went to our beach, or fishing hole, it took a while to get there. Nevertheless, these were the happiest time of my entire life. Here I was, in a boat, living the dream.

When Grandpa traded the old kicker in for a 10 HP twin cylinder (and made the boat plane) I couldn't imagine that a boat could go that fast.

Later in my youth I had a friend with an 18 foot lapstrake fitted with a 75 HP outboard, and it was a downer. He always insisted on going WOT and it was very uncomfortable.

With a young family we had a tinny with a 9.5 Evinrude. It would slightly plane (LOL) but we had as much fun as I did as a youth.

If speed is your requirement, invest 50 or 60 grand and maybe you'll be satisfied. If you want family fun, you'll be just fine.

Prop your motor right and you won't hurt it. Remember, the rule of thumb is that at minimum a boat only needs between 25 and 40% of the rated HP.

 

[ahicks]

Agree, if maintained and propped properly you aren't going to hurt that motor.

 

[roffey]

As a kid I had a 8 foot hydro with a 10hp. That boat would go about 30mph and it felt like 100mph. The boat and speed are in the eye of the beholder. I think when on the water the journey is what its all about. Once you arrive the boat ride is over, lol.

 

[Texasmark]

As a kid I had a 8 foot hydro with a 10hp. That boat would go about 30mph and it felt like 100mph. The boat and speed are in the eye of the beholder. I think when on the water the journey is what its all about. Once you arrive the boat ride is over, lol.

Right smack on bro. Ha! I had the same thing, old wooden stepped hull hydro, had to sit on your knees, squeeze throttle, cable steering and the cables and anchors for them were outside the boat. Engine was a 1955 or '56 Scott Atwater 10 dad bought new.

I didn't have a trailer and it was light enough for me to slide in the back of my PU, my first truck....47 Chebby, hot water 6, 3 on the tree, fully worn out when I bought it but it was a truck and mine and it would carry my boat and life was good. I had worked as a deck hand on a tug boat that summer and had some money to spend. I wasn't so optimistic on my speed, figured actual about 25, years later, and felt like 50 at the time.

 

[Sea Rider]

If wanting to get the max fun and water performance out of that combo, the motor must go inevitably for a prop maximization to a less prop pitch from factory delivered one. If the motor is brand new, soon after break in period ends install a Hardline Hour/Tach model HR-8061-2, go for a wot run as will be loaded, with deck load perfectly distributed while the combo runs on flat calm water cond. Report the max wot rpm achieved to go from there...

It's a day/night boating difference running a factory delivered prop with motor against a maximized prop for your specific boat.

Happy Boating