Aluminum boat planing question

[reelfishin]

I have a neighbor who has a 14' Starcraft Sea Scamp, 14' with a 20 hp max and 700lb rating. He's been running an old 3 HP air cooled motor. I decided that rather than deal with his too small motor today, I'd take my Merc 110 which I run on my 15' jon boat. My boat weighs in at about 575 lbs with two pedestal seats, wood deck and rod holders, and bow mount trolling motor. The little Merc runs that boat at about 18 mph on the GPS with both of us in it.

I figured that in his bare bones V hull it should do better.
We had the two of us, myself at 300lbs, him at about 200, the motor and a 3 gallon fuel tank. The boat is light enough that I can lift it myself, it can't be more than 200lbs or so without the motor.
To my surprise, I couldn't even get the boat on plane with the 9.8 HP?
His boat is about the same width, lighter, and I would think, far easier to push through the water. Both are flat bottomed at the stern. RPMs sounded about the same as on my boat, but the motor stopped just short of getting on plane. Another mile or two per hour and I feel it would be on top, but it labored something terrible. My bass boat is rated at 35 hp, and is too heavy to carry, this thing is light. I even tried a tiller extension I have and it made little to no difference. His boat has no flotation, no floor, no place to hide weight. My Merc 110 has the original 2 blade prop, I have a lower pitch three blade prop I tried on my boat but it gave me too many RPM's at cruise.


I don't get it? I didn't have my GPS with me, but I'd guess it was no more than about 10 or 11 MPH. Regardless of trim, the boat would just plow bow high sinking the stern lower the harder I pushed it. It did the same with him at the tiller and me up front. It felt like I was towing the anchor along the bottom. There were times when the breeze kicked up where I felt like I was standing still. The wind was minimal at best. I can't figure what on that boat has so much drag? I have an older version of that same boat, and the same motor flies on that boat. My older version has more runners on the bottom than his does, but otherwise its the same boat.
I would have bet money that my 9.8 hp would have gone at least as fast as it does on my boat, if not faster. I don't think we were going any faster with the 9.8 than he goes with his old Sea King air cooled motor.
The motor was running good, has perfect compression, and has very low hours on it.

The boat don't look or feel overloaded, at rest it sits fine in the water. It drops in the stern about 6 inches under power. I tried the lower 4 trim pin locations and the lowest was the best. The motor appears to be actually trimmed in a bit looking at it. Which I would think would lift the bow.

 

[CATransplant]

Re: Aluminum boat planing question

I have a 12' Sea Scamp and it will barely plane with my 6 hp outboard, and I've worked long and hard to get the weight distributed just so. It won't plane at all with two people.

I say that because, my boat is rated for 10 hp, so I have 60% of the rated horsepower.

On yours, which is rated for 20 hp, you have just 50% of the rated power. Along with that, you've got two big guys in the boat, plus gear.

Those Starcraft boats aren't exactly planing whizzes, frankly. They're a compromise between a row boat and an outboard boat. The V-hull part of the hull does tend to ride low, and that's good for rowing. For motoring...not so much.

Lifting the v-hull part of the boat can be tough if the boat is underpowered by much.

A 15hp would run that boat at about 16-18 mph with the load it's carrying. A 20 hp would run it about 24-26 mph. A 10 hp? It might plane it at 14-16 mph with one person aboard and good weight distribution, or it might not.

Your other boat, apparently is an easy-planing hull. The Sea Scamp isn't.

 

[David Greer]

Re: Aluminum boat planing question

I'm guessing your two blade prop was also a factor (I have one on a 200 that replaced a 3 blade on an 18 OMC--it's quick but the old one had the power.) But as was said, might not plane at all. Test with one of you?
You can run over shallow water to use the water pressure to get up, too.

 

[film495]

Re: Aluminum boat planing question

Is the shaft the right length with the cavitation plate at the right level? Also, have you adjusted the angle of the motor? - that might help. The prop might also help you if you try another maybe 3 or 4 blade prop - maybe too a hydrofoil I have heard can do magic with small boats. For fun - if you really want to play around - you could flip the thing over and put a coat of automotive wax on the planing area - and that will certainly create less drag and you'll go faster, but I have never done that - so, I am not sure if it is advisable - safe - or environmentally friends, but it should help go a little faster.

 

[reelfishin]

Re: Aluminum boat planing question

I thought about a hydrofoil but for one, the motor is on my boat 99% of the time, and I also figured it would drop the speed even further adding even more drag. With the tiller extension on, and me sitting on the middle bench, the weight is all out front, and the boat still rides bow high even with the motor trimmed all the way in.
I can't put any more hp on either boat since we have a 10hp limit on that lake. In the river or back bays, I run my 15' Grumman V hull with a 20 hp which does just fine with three guys, that boat planes out nearly instantly.

Another concern about using a hydrofoil is that the motor is a clamp on only, all that weight would be relying on two motor clamp screws.

I would have figured that the Starcraft would have far less resistance to overcome in the water than my bass boat, not to mention it's so much lighter.
We had no gear in the boat, just two guys, a 3 gallon tank, and the motor.
I think it may have been worse with just me in the boat, it would have been heavier in the stern with only one guy. It had no problem getting the V part of the hull up out of the water, it was the last half of the boat that wouldn't lift. I was actually pretty surprised to see how much the boat sank in the stern under full throttle. It would dip low enough to wet the bottom of the shroud. The cavitation plate was about 5/8" above the bottom of the boat.
The boat also created a huge wake, my old 31' bay boat didn't make that much of a wake.
His Starcraft is older than mine, mine is a 1982, his is a 73, the only difference is that mine has two more strips along the bottom and mine is 'well used' to say the least. Both are within a few pounds of weight, both are the same width and length. I think I may try my 20 hp on his boat just for comparison, but something tells me it may be worse since the 20 hp is so much heavier and it will no doubt need more fuel, meaning a 6 gallon tank of gas. That added weight my put the stern even lower. Being a bench type hull, moving the tank forward is tough, there's no way to really route the fuel hose other than over the benches. I had my longest fuel hose on the 3 gallon tank which put the tank just behind the front bench but that put the line right down the middle of the boat but I was trying to see what would help. With just the motor and tank in the boat, the boat floats with the drain plug just barely submerged, with my in the stern it sinks about 6" with a front passenger, thats reduced to about 4". The boat don't feel or look overloaded. I am wondering if the simple fact being that my other boat is a flat bottom, means that the boat is just not as deep in the water to begin with. Even though there's more drag or water surface on the hull, the flat hull is supporting the weight better. While his boat is basically flat in the stern, and most of the way back, it still has some curvature and the edges are rounded, where as mine are square.