little 14 ft starcraft wont go faster than 6 mph!. (not engine spacific)

[BlackoutBill21]

So I'm posting this here because I buy and sell outboards for fun. Every.. EVERY motor I have fixed and tested will not get boat faster Than 6 maybe 7 mph.

Im starting to think its boat related. It is a Deep V hull 20" transom. 14 FT starcarft. I started to think I need a longshaft!! so I bought one.... same result! The biggest motor I have is 18 HP short shaft and it gives the most power,(boat nose lifts highest) but still only 6 mph. the 9.9 longshaft hit a whopping 8mph for about 1 second.

Hull looks pretty decent, straight, no huges gashes... could all my motors (being shoft shafts) be one issue, then the one long shaft I bought have a spun prop? what are the odds?

Let me know if you guys think its boat related. If not I'll buy a tach and start some more in depth motor testing... i just have a hard time believing 20 motors later and they all have some sort of issue. lol

 

[fhhuber]

Several possible causes...

Too heavy for the HP. If its too heavy you'll never get it on plane.
Bad engine angle. Start with prop shaft very near parallel to the keel at the transom.
Prop too small diameter.

14 ft and 18 hp as the biggest you have tried = I expect you just don't have the HP for the weight of the boat and stuff in it. Remove as much weight as you can.
15hp can get a light loaded Jon boat on plane if its lightly loaded. Your Deep V is going to need more.

 

[BWR1953]

Fiberglass or aluminum hull?

 

[BlackoutBill21]

Thanks for the responses! It's aluminum! And it's just a fishing boat. So it's only me the motor and my fuel tank lol I didn't think this boat would be that heavy but I get it. Yeah the boats only rated for 30hp max and I just haven't had any come across to buy that high. There isn't much I can take out of the boat to lighten the weight aside from getting my 198lbs to the gym. lol

I will see if I can try a bigger motor though

 

[ahicks]

Sight along the bottom, looking from the back forward. If it's caved in slightly, it's going to be tough to get it to plane out.

 

[fhhuber]

You want a minimum of 75% of rated max HP.... or about 22 HP Preferably 90% of rated max to rated max.

 

[Scott Danforth]

6mph is hull displacement speed

Is your 18hp in tip-top shape? Good compression, etc?

with your 18hp, move as much of your gear to the bow, sit in the middle and go for a ride. It sounds like youre aft heavy and the motor doesnt have enough power to climb out of hole

 

[Watermann]

You don't have a speed boat designed hill that's for sure and without any pics to see it's tough to tell if you have an issue. Off hand I'd say you're not powering it with what it needs. A 20" transom on a 14'r is out of the norm too, most are 15". A healthy 25 HP motor should jump that 14'r up on a plane nicely.

 

[flyingscott]

What model starcraft is it.

 

[BlackoutBill21]

again Thank you for all the responses. Now If i find a 22 hp does it need to be long shaft? Also I will try the shifting weight and check the bottom for caving. Not sure if i'll be able to steer from the middle though lol

not sure the model. here is a video of it with the 18 hp on the back... TIP TOP condition? i believe so. there are videos of it running if you wanna look at it. I changed the prop to a bigger pitch to make sure it wasn't spun. I do see the possibility of being aft heavy though. The deep transom and water has come over the back for a little swamp action on heavy wave days,

should I look into shortening the transom?


https://youtu.be/R39wVZIe7Gw

 

[southkogs]

I would think that Starcraft is a little heavier than you think it is. I've got a 14' open row-boat style, and we run a 9.9HP on it. It get's up on plane, but ain't real fast. 18HP is still 50% of max on your boat. I would think the 18 would push you up on top, but it'd be a light load and trimmed just right.

 

[jbcurt00]

No idea whats the primary problem, w the limited info provided. But whatever it is, shortening the transom wont be the solve.

I have a 12ft Sylvan open aluminum fishing boat, that is essentially an SC clone. The 1970 9.5hp Evinrude pushes it plenty fast enough on plane at WOT w just me in it, that I wouldnt want to try a tight turn at WOT on anything other then perfect, flat as glass, water. Dont know what speed that is, but is above 6mph, well above it. And although it runs well, I wouldnt say its in TipTop shape (running perfect and reaching the correct Max RPM range) either.....

 

[fhhuber]

When you are having trouble getting up on plane, more pitch (probably with less diameter) is not the right answer.

You probably need about 10 to 12 mph to get that hull up on plane. Until you get there, more pitch (trying to make the boat even faster) isn't ever going to help. The prop can be TRYING to push the boat 40 mph and never get you on plane if you don't have the THRUST to push it up.

Probably with a low enough pitch and large enough diameter prop even a 15 could push it up on plane... (and that's about as fast as it would go)

You'll get only so high on plane if you don't have the long shaft outboard the hull "wants"

Anti-vent plate should be very close to even with lowest point of the hull at the transom.
If the shaft is even with that lowest point (or higher) you have the hull preventing the prop from pulling the water correctly to provide thrust. You may not be able to get on plane like that even with a 35 hp outboard....

You can get or make (better to buy one) a "jack plate" to push the outboard back and allow it to be mounted lower than if you hang it direct on the transom. This is pretty unusual for this small of a boat... and you'll have to push weight, such as the trolling motor battery forward so the boat will ride properly if you move the outboard engine back.
Cheaper than buying a new engine and a much better choice than chopping the transom.

 

[roscoe]

If it is a 20" transom, you will need a long shaft motor.
But it lookalike the prop is well below the hull in the video, but hard to see clearly.

NEVER shorten the transom, you will ruin the boat for all eternity.

That boat should easily plane with a 15 hp.
Had a similar 15' Lund that kind of planed with a 9.9, but easily planed with the 15.

 

[flyingscott]

Measure your transom

 

[BlackoutBill21]

I want to thank y'all again! Ok I won't shorten transom lol I don't know what more info is needed for the one who said he had limited info.i gave transom Hight motor sizes Boat type and videos...but whatever,

I will play around with all your great tips! Thank you!

 

[Watermann]

That is really a nice 14' SC hull, in fact I would to have one, it would make a super small lake fishing boat.

I think the era of the motors you're using have the most to do with performance or lack there of with your hull. Although I didn't get to see the main motor run in the video. After seeing your videos it looks like you rescue and use old motors from the 50's and 60's. If you were to install a healthy 25 HP OB from late 70's up then you would see a world of difference in performance of that boat.

 

[BWR1953]

If your engine is running right and is correctly propped, it should put that little 14 footer on plane, no problemo.

I used to have an aluminum 14' Sea Nymph similar to your boat that had a 25HP Mercury on it and with 3 men on board it performed very well, hitting 25-30MPH without effort.

Even my current 16' SC Kingfisher, a larger and heavier boat than yours, would semi-plane at 8-10MPH with my big carcass plus my wife and boy in it using only a little 8HP tiller Johnson.

Like so:

Here's how you measure for the correct outboard shaft length. It needs to match the hull. Too long or too short is not good.

 

[JimS123]

I have the same hull with a 25. I get awesome performance. With an 18 you should get at least 25 mph.

Get the kicker off your transom - you don't need the extra weight.

From the pics it looks like you have a long shaft transom. But I donno. Just use a tape measure - its easy.

 

[fishin98]

I have a similar boat, Alumacraft Lunker V14, rated for a 35hp. It also takes a long shaft(20") motor. I have a 1999 15Hp Merc with a 9P prop on it. With one person and gear it will do 19mph GPS indicated, will plane at 12mph. With 2 Large people it will do 12-13mph and barely plane. With my 2007 Merc 5hp 4str it does 6-6.5 mph GPS indicated, with 1 or 2 people. I plan on a new Suzuki 20-25HP this year. Those old motors you have hanging on the back were not know as speed demons. From the looks of your video..you require a LONG SHAFT motor. Depending on what your requirements are..a 15-30hp Long shaft motor is what you need.