why won't my boat plane?

[ericman]

https://www.tinboats.net/forum/viewtopic.php?f=21&t=30830

I did this build a few years ago. Love the boat. Rows great. Stable, yet narrow. Great for motorless lakes. Started hunting the Mississippi River a couple years ago and was a little concerned about hiding my big boat so we hunted out of this with the 3 horse. Long story short. Very long trip back to the landing (upstream). Bought a 7.5 merc. The fin on the lower unit is dead even with the very bottom of the boat. Note that the hull isn't square at the back, it rises up the last 18 inches or so. Again, the fin is dead even with the very bottom, not the bottom at the back. The propeller IS completely under the bottom of the boat. With just myself and 1 other person, it WILL NOT plane. We both weigh 180. Also note how big the center keel is right at the back of the boat.

Is it possible that this part of the keel is diverting so much water that the prop just doesn't have enough water to push. I don't really hear a washy sound like you can if the prop is just up too high. I also just had a new bushing pressed into the prop because it would wind out with one little blade of grass so that part of the equation should be fixed.

Any input would be appreciated.

 

[Shaugh]

You probably just don't have enough horsepower to plane out. My 16 ft alumacraft won't plane with just me in it... with 2 people forget it.... it moves but it will never get going fast enough to get out of the trough...

 

[ericman]

You probably just don't have enough horsepower to plane out. My 16 ft alumacraft won't plane with just me in it... with 2 people forget it.... it moves but it will never get going fast enough to get out of the trough...

I've had an 8 hp evinrude on a 16 foot Lund Bench boat with 2 people plane just fine. That boat was rated for 20 hp. This little Alumacraft is rated for 5.5.

Any other ideas?

 

[Shaugh]

A longer boat will plane more easily with the same total weight. A small, narrow boat that is heavily loaded starts out deeper in the water and has a smaller wetted surface. That requires more energy to get up and stay on plane.

 

[Stumpalump]

I think it may be the hull with that rounded rear section sucking it down.

 

[ericman]

I think it may be the hull with that rounded rear section sucking it down.

So, no matter what I do, it will do this?

 

[nccatfisher]

You have way more boat/load than you have motor.

 

[ericman]

You have way more boat/load than you have motor.

Again, it's rated for 5.5. I have a 7.5 merc that runs awesome. I have used an 8 horse evinrude on 16 foot lunds rated for 20 hp with 2 people that planed just fine. This boat is 13 feet 9 inches long and 46 inches wide. It might weigh 120 pounds. Myself and someone in the front might weight 350. When I used the 8 horse on a 14 foot sears boat it went close to 40 mph.

 

[jethro]

Some hulls just aren't as efficient as others. With a max HP rating of 5.5 I'd say that boat was never designed to plane in the first place. That boat has no beam to give it surface area. You might do better with a hydrofoil to give the motor some lift, but that is a shot in the dark as far as I am concerned.

 

[Stumpalump]

I think it may be the hull with that rounded rear section sucking it down.

So, no matter what I do, it will do this?

Basically yes. Jethro's hydrofoil may do it. I make my own out of thin aluminum plate but I was thinking a plate on the bottom of the boat to square it up would work. Might not be as hard as you think. They used the same idea on square back canoes with outboards to get the speed up.

You would just need a flat sheet as wide as the stern and bent at a 90 to rivit to the transome. The metal getting place could cut and bend it cheap enough. It would not have to be a watertight compartment and would drain as soon as you were on plane.

 

[RiverBottomOutdoors]

Not enough HP and the hull is not designed for it either. What you have is a row boat that you can put a motor on to supplement rowing. It was never meant for planning speeds. That is why it has the oversized keel that acts like a skeg; to keep the boat tracking when rowing.

 

[nccatfisher]

You have way more boat/load than you have motor.

Again, it's rated for 5.5. I have a 7.5 merc that runs awesome. I have used an 8 horse evinrude on 16 foot lunds rated for 20 hp with 2 people that planed just fine. This boat is 13 feet 9 inches long and 46 inches wide. It might weigh 120 pounds. Myself and someone in the front might weight 350. When I used the 8 horse on a 14 foot sears boat it went close to 40 mph.

I need to get your formula for getting the best out of a motor. I have a 1448 with a 25 HP on it and I weigh 190# and I get 23MPH by the gps.

 

[ericman]

It seems that maybe the bottom of the hull rising back up to the transom might actually be shooting water right past the motor, preventing enough water from getting to the prop to "jet" me forward. I'm going to make a homemade hydrofoil and try that out. I may also make a 6 inch wide center mounted trim tab for the bottom of the transom if that does not work. I've read several testimonials about hydrofoils on forum.iboats to at least see some promise.

Thanks for all your input.

I will let you all know what my results are next week.

 

[Stumpalump]

That has got to help. What needs to happen on most boats is a clean break from the surface of the water at the transom. A rounded corner keeps the adhesion and sucks the boat down. For example glass boats are made in a mold so a perfect 90 degree corner from the bottom to the transom is not possible. It is ever so slightly rounded to get it out of the mold. On an expensive go fast boats the factory or the bottom blueprinter will grind and reshape this edge to make it as sharp as a piece of cut steel. That alone gives at least a mile or two increase in speed. You are not going 60 but the same principle applies and since your rounded edge is extreme that is a problem. Will a hydrofoil lift the transom hard enough to break this suction? Idk but that's what I'd try because it's easy and this is not a 60 mph boat so who knows what rules really apply. You have closer to what would be called rocker. Rocker/hook/rounded transom all create drag and the faster you go the more it creates. Good luck making sense of all the secretive confusing hull info if you dare to wade into it.

 

[Crazyboat]

So the OP's boat planes with 1 up but not 2 up, am I understanding this correctly? Try adding weight to the boat with just 1 person onboard and see where the engine reaches it's limit to plane.

I'd drop a 10 on back and be done with it, but maybe you feel it's not strong enough, I don't know. Personally pushing 2 guys and gear with a 7.5 is asking quite a bit. I had a 7.5 when I was a kid and I had to shift weight around when my dad rode with me, he was 170 and I was maybe 140. 12' boat that weighted 80 lbs dry.

 

[surfman]

Hull is not designed for planing.

 

[ericman]

So the OP's boat planes with 1 up but not 2 up, am I understanding this correctly? Try adding weight to the boat with just 1 person onboard and see where the engine reaches it's limit to plane.

I'd drop a 10 on back and be done with it, but maybe you feel it's not strong enough, I don't know. Personally pushing 2 guys and gear with a 7.5 is asking quite a bit. I had a 7.5 when I was a kid and I had to shift weight around when my dad rode with me, he was 170 and I was maybe 140. 12' boat that weighted 80 lbs dry.

I never said the boat planed at all. Result is the same whether myself or someone in the very bow of the boat. Again, if you read my post, you'd see that an 8 horse evinrude pushed a 16 foot Lund rated for 20 horsepower quite nicely with 1 OR 2 people. SO, it would stand to reason that 7.5 horse should be more than adequate horsepower to push a 13' 9" boat that is only rated for 5.5 horse.

 

[benjineer]

I think it should plane, at least with just you in it, but I really don't know the effect of that rounded bottom at the back. I would say check your tilt pin and experiment with different angles there. Typically you plane best with the motor all the way down, but it won't be efficient with the bow plowing down. In your case it may be worse with the whole boat plowing and not enough stern lift. You may have to tilt it up more in line with the angle of the bottom and maybe even raise the motor where the cavitation plate is more in line with the cut off point. It does stand to reason that the boat would want to sag in the back and run deeper because of the reduced buoyancy in the back. Just my opinions. I wouldn't give up on the 7.5 until I had experimented with the angles and position a little more.

 

[jethro]

I never said the boat planed at all. Result is the same whether myself or someone in the very bow of the boat. Again, if you read my post, you'd see that an 8 horse evinrude pushed a 16 foot Lund rated for 20 horsepower quite nicely with 1 OR 2 people. SO, it would stand to reason that 7.5 horse should be more than adequate horsepower to push a 13' 9" boat that is only rated for 5.5 horse.

You are getting hung up on the size of the boat and the horsepower of the motor. A skilled naval architect can design a 50 foot boat that could plane with a 2hp motor. Fact of the matter is you have a displacement hull that was not designed to plane.

 

[Shaugh]

You can make a trash dumpster plane if you put enough horsepower behind it. But in that boat you are hampered by it's small wetted surface area and a marginal amount of horsepower.

A 7.5 hp motor is on the borderline to plane a boat with 400 lbs loaded in it. I recall my youth and a 12 ft fiberglass Sear Gamefisher with a 7.5 McCulloch ... a heavy pig of a boat, but I could get it to plane if I left the motor and went and hung half my body over the bow for a few seconds.. then I could go back and it would stay on plane. With my 90 lb brother in the boat it would not plane.. ever. There is no magic to getting a boat on plane, but you do need enough surface area and horsepower for any given weight. Adding a fin in back could do it because it will increase your surface area.. that would be the least expensive route... either that or a bigger motor.