Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

[lancew]

Hello!

I recently got my first (engine-powered) boat - an 8'10 inflatable. Has a soft floor with a few wooden slats fitted to it for rigidity. It's powered by a 4HP Tohatsu 4-Stroke outboard. The outboard is brand new - about 8-10 hours on it. I have not gone WOT on it yet, keeping it around 1/2 throttle at max.

I can get it up on plane well enough with just myself and some gear (gas can, small cargo box, a couple of lifejackets and a seat pad) however cannot keep it on plane. It seems that once on plane for 30 seconds - 1 minute, the engine will suddenly stop spinning the prop and rev up really high. More than once it has almost thrown me out of the boat/off balance. Once the small wake behind the boat hits the engine, the prop will kick back in and I'll accelerate again, but no longer planing (since the speed died so quickly/drastically).

I'm assuming this is due to the boat/engine cavitating, but not sure why the engine would stop the prop and rev up. I'm assuming that the prop stops also, based on the behavior of the boat - I have no way to look. If I shift my weight to the back, the prop will engage again. I cannot get it on plane while sitting at the back to begin with.

The prop/cavitation plate is a few inches (4 or 5) below the bottom of the boat while the boat is at rest, due to the rather smallish transom on the boat.

Would this be due to the engine not having enough water to suck in to cool itself (because of cavitation), so as a safety it throws it in neutral/stops the prop? If so, what would be the solution for this - lowering the motor (also known as cutting the transom)? Could it be something wrong with the propeller?

This happens on rough and calm waters (though reaching plane on the ocean/with waves is more difficult).

Help please and thanks!

 

[Bronlonius]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

It sounds like the outboard is mounted too low, can you post a picture? I'm not an expert at inflatable boats, but in general your cavitation plate should be within an inch of the bottom of the boat with an aluminum or fiberglass boat.

 

[lancew]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

If that were the case, would it simply be building up the transom with some wood to raise the prop?

I'm having a hard time imagining the physics behind this - would you be willing to paint me a word picture as to how a lower prop would result in this behavior?

I'll get a pic together tomorrow afternoon - I don't store the boat with the outboard on it.

Thanks!!!

 

[Bronlonius]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

Not sure if I can explain the physics of this correctly, but basically what you will experience is the outboard "digging a hole" in the water if the leg is that deep. The prop is supposed to run right up by the surface of the water, and from my experience of trying to run a 20" shaft on a 15" transom the prop seems to pull the surface of the water down when it's too deep. You end up with a really big wake, and a really slow boat the eventually blows out the prop because it can only keep pushing that water down for so long. You also have the added drag of the leg in the water - if you notice, it's not a sharp edge above the antivent plate.

What is your current transom height? It sounds to me like you probably have a 15" transom and a 20" motor. If that's the case, the easiest solution is to get an outboard with a 15" shaft. If you have 15" motor and the transom is really only 10-11", then the only solution is to build up the transom to the correct height. Again, I don't know much about inflatable boats, so I don't know if the transom is strong enough to handle this or not, but on rigid boat, this is usually accomplished with a jack plate. You can buy the "mini jacker" for about $100, which is a fixed height plate, or build an adjustable one out of aluminum angles like I did. Or maybe the transom board on your boat is removable, and you can simply replace it with something taller? Pictures would definitely be helpful.

To give you an idea of what correct motor height looks like, there are pictures of my jack plate setup in post #12 of this thread:

http://forums.iboats.com/prop-quest...-nissan-15hp-2-stroke-615276.html#post4290915

... I don't think you need to go to the same extremes that I did with my jack plate setup, but you do need to make sure that whatever you do is strong enough to hold the motor.

 

[Sea Rider]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

A 15 HP and a rigid boat can't be compared to a tiny slatted deck with a 4 HP at back. Both are different boat types. Assuming engine is a short shaft, sitting at proper transom height and runing at optimum state, it's a size/HP related issue. Slatted tender sib too small and engine too poor.

Happy Boating

 

[colbyt]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

I have a new 3.5 and the manual clearly states that the ideal height is for the cav plate to be no more that 1.2 - 2" below the bottom of the boat. Honestly I don't know enough and haven't run mine enough to tell you if that is the problem or not. I suspect it is. The flexing of your soft hull may also be a contributing factor.

I can tell you that mine is level with the bottom of the boat (contrary to manual) and it runs great at 50%. I need a few more housr for WOT. I also never expect to plane a 14/48 Jon with 600 pounds of lard in the seats.

 

[Sea Rider]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

What engin'e owner manual states, is just a refference. You should start placing your engine at back transom and under error and trial raise or lower engine accodingly till the best engine height performance has been reached. If at wot an inflatable dosen't achieve full plane, just fast displacement speed, don't lose precious boating time, no matter what you do will be useless. A sib with lower inflatable keel is much easier to plane than a flat slatted deck, but both still have Size/HP limits.

Happy Boating

 

[lancew]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

Looks like the cav plate is just over 2.5" below the transom. I have begun construction of a small jackplate to rectify this - hopefully it works and gets rid of the prop cutting out at plane!

 

[colbyt]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

Pictures are notorious for not clearly illustrating angles. Looking at that picture I want to say you need to read the trim section of the manual. For in the perceive position now when you put weight in the back of the boat you are going to act like a porpoise.

The vertical shaft of the motor should be 90 degrees perpendicular to the water when loaded, seated and moving.

 

[Bronlonius]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

This looks like it should be solved fairly easily.

With only 2" or so to go, it should be easy to raise the leg. Be sure to experiment with trim angle with each change. I would start by putting a piece of wood like a 2x2 deck spindle or something between the motor and top of the transom, assuming you can still get the clamps to clamp on to the transom. Once you've determined the correct height and trim angle, you can then do something more permanent.

 

[pvanv]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

Your boat should be powered with at least 80% of the max HP in order to give acceptable performance. Also, the motor looks too high (sorry, I meant LOW). It needs to be at a jack height where the water passes between the antiventilation plate and the splash plate. Likewise, it must be vertical when at speed.

 

[lancew]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

Your boat should be powered with at least 80% of the max HP in order to give acceptable performance. Also, the motor looks too high. It needs to be at a jack height where the water passes between the antiventilation plate and the splash plate. Likewise, it must be vertical when at speed.

It being too high seems to go against what everyone else is saying - could you elaborate please? Everything else I've read says the anti vent plate should be level with the bottom of the transom.

 

[Bronlonius]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

The Cavitation plate is definitely below the bottom of the boat in that picture, It needs to go up.

 

[TOHATSU GURU]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

The engine is too low on the transom, the engine needs to be converted to a six hp and if the boat does not have an inflatable keel it was not designed to get on plane at all.

 

[lancew]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

The engine is too low on the transom, the engine needs to be converted to a six hp and if the boat does not have an inflatable keel it was not designed to get on plane at all.

I made a jack plate, raised it 2.7 inches. The problem was twice as bad. Removed the jack plate, and its back to where it was. I'm thinking it needs to go lower.

 

[Bronlonius]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

Based on what the other guys (who know a lot more about inflatable boats than me) are saying, it seems that there are 2 problems:

1. you're underpowered
2. you're boat doesn't have an inflatable keel, so it's not designed to plane.

If I interpreted Sea Rider's post correctly, he's saying that if the motor height of a SIB is correct and you can't get it on plane at WOT, that there's not much you can do. However, Elvin makes a very good point that the 4HP can be easily converted to a 6HP. That's probably the next place to try.

 

[lancew]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

Well, it's not that it won't plane - when it gets to plane, the prop cuts out and the engine revs, almost as if the hub is shot. But the engine is brand new. I really don't want it to be that.

 

[Bronlonius]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

I wouldn't rule that out. Maybe the hub spun from running the leg too deep, or maybe it was just bad to begin with. New doesn't always mean good. If you mark the inner and outer section, then run it you'll know for sure if it's spinning the hub if the marks aren't still lined up.

 

[Sea Rider]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

Being a double post said earlier to pass that 4 HP into a 5, but it's suggested to 6 HP, that's even better, just a carb swap. Anyway to avoid losing precios summer boating time, why don't you perform the prop hub slip test, will only take at the most 15 minutes, latter will know for sure if prop is spinning in top condition. When engine bogs down, is the sib flexing ?

Happy Boating

 

[TOHATSU GURU]

Re: Trouble planing with 4HP Tohatsu 4-Stroke - new to this and need help!

I made a jack plate, raised it 2.7 inches. The problem was twice as bad. Removed the jack plate, and its back to where it was. I'm thinking it needs to go lower.

I've seen small keel-less inflatables get on plane. Usually with someone less than 55 kg as the driver, regardless though, the engine is too low on the transom. Generally the cavitation plate should be anywhere from 1" below to 1" above to be correctly positioned for peak performance. Most perform better slightly above the transom, however, as keel-less inflatables are not designed to get on plane...