Repower Big Jon and Prop Question

[Ranger354]

Had a friend put a 115 Yamaha on his 20ft Aluminum Jon. I forget the make of the boat, but it is wide and long. 8ft wide by 20ft length. This boat ran fine with the 70hp evinrude on her. She was just a bit slow and the motor was getting unreliable. So he purchased a 115 Yamaha (used) to put on the big girl. We have been out numerous times and just can seem to get the set up right on this boat. The 115 had a 13 1/2 x 17 on her at first and this gave the boat a great hole shot, but at top end the boat would run over 6000 rpms and the bow of the boat would dig with water eventually splashing or spraying over the front.

The boat has a 4 inch manual jack plate and we tried it at all different heights. He has borrowed 3 additional props to try ( 19, and two different RAKED 20s) and other then the rpms coming down into range (5200 to 5800) the boat acts the same. Let me tell you it is a prety wild ride at 40mph and the boat is surging and trying to dive.

I have operated all sorts of bass boats, flats boats, even jon boat and never had this problem. The motor CAV plate is level with the bottom of the boat like it should be when starting.

Even when we try and trim to motor at a mid range of 3500 rpms we get almost NO bow lift.

I am at a loss. I thought for sure we would have this boat figured out by now.

Could additional cupping help that much?

Any info would be much appreciated.

Jim

 

[mommicked]

Re: Repower Big Jon and Prop Question

Is it a flat bottom jonboat and how much does it weigh?

 

[Ranger354]

Re: Repower Big Jon and Prop Question

Sorry, but don't know the weight.

 

[steelespike]

Re: Repower Big Jon and Prop Question

I would guess it has a hook in the bottom.Lay a straight edge along the keel near the stern.
Is it rated for a 115?Do you have a hydrofoil on the motor?

 

[Ranger354]

Re: Repower Big Jon and Prop Question

The rating or ID plate is not on the boat any longer. I thought it might have a hook, but have had he boat upside down repairing the bottom and did not notice a hook. The boat handles the 115 with ease and performs good at 3500 elms.

 

[Ranger354]

Re: Repower Big Jon and Prop Question

That is 3500 RPMs... Darn auto correct phone.

 

[steelespike]

Re: Repower Big Jon and Prop Question

With the boat almost diving and unresponsive to trim I would still wonder about some sort of distortion of the hull.Does the boat behave as it did within the speeds of the 70 hp?

 

[Ranger354]

Re: Repower Big Jon and Prop Question

Best speed with the 70hp was 28mph or close. With the 115 I saw 35 but she never got running good due to the diving problem. Hard to watch gauges when you have water in your face!! The 115 does have a whale tail on it also and we have tried it with the WT removed. Will do so detail measuring and checking hull and also plan to install electric jack plate so the engine can be lowered more or raised while driving.

 

[steelespike]

Re: Repower Big Jon and Prop Question

With the boat almost diving and unresponsive to trim I would still wonder about some sort of distortion of the hull.Does the boat behave as it did within the speeds of the 70 hp?

What I was looking for was does the boat with the 115 behave the same if you drive the 115 at the 70s speed? Does it seem to respond to trim at 28. does it seem like the bow is a little stuck down compared to your trypical boat.Keeping in mind the 115 is almost 100lbs heavier.

 

[Ranger354]

Re: Repower Big Jon and Prop Question

It does not respond to trim but acts about the same as the 70 did. NO diving at slower speed. I still think this motor needs to sit lower in the water. Since is will BLOW out very easy when is trimmed just a little. The new plan is to swap jack plates and install the elec. with the 115 lower in the water with the plate down. Then raise it while driving to get the right height.

 

[mommicked]

Re: Repower Big Jon and Prop Question

Is it a flat bottom or a shallow V? I had blow out issues on an 18' shallow V hull jon but no submarineing. Switched to a 4 blade prop and all performance increased a great deal. Hole shot is unreal now. My F90 motor weighs 370lbs.

 

[Ranger354]

Re: Repower Big Jon and Prop Question

It is a big flat Jon boat, no V. Had a prop guy put some additional cupping in the prop and we shall see how it works.

 

[Ranger354]

Re: Repower Big Jon and Prop Question

Turns out the hull had a hook in it. Constant riding on running boards that didn't extend out past the transom bent the aluminum hull between the last cross rib and the transom. There is about 1 foot of gap in there that was pushed upwards. Took a piece of wood and big hammer and tapped it back flat. Runs like a scalded dog now!

 

[steelespike]

Re: Repower Big Jon and Prop Question

Its allways nice to see a success;thanks for giving us the update.
Hopefully it won't oil can on you.
I assume you extended the bunks on the trailer.