New custom landing craft performance problems.

[kengrome]

It is very pronounced when the wheels unload, not gradual. If we try to go 8 or 10 knots while rolling in a 2'-3' chop as the boat rolls and one engine will rev then bite then the other lets go back and forth with the roll, very irritating. I'll see what they think about that.

tkinak, you never mentioned this prop unloading when rolling side to side until now. This sheds a whole new light on the situation. In this case the air is most certainly coming in from the sides. This suggests the engines need to be lowered so they don't get so close to the surface when the boat rolls.

Basiclly you need to find a way to keep the props deep enough in the water so they won't suck air. Those fins above the props will help, but so will lowering the engines.

[Manie B]

Hi me again. Extending the hull plate to the cav plate doesnt do much. When we built our jacking system for ski racing - it already pushed the engine further away - no difference. That is why end "brackets" work well. I found this on the net - nice stuff, look carefully the bottom of the bracket is higher than the hull - motor depth is important. The pics are small but you get the idea.

My buddies cat has 3 bunks in the hulls the fourth one is the toilet - door is visible on one pic. The bunks are comfortable for 1 cosy for 2

[Manie B]

Sorry here is the brochure

[tkinak]

Does anyone make a mechanical jacking plate? the old screw type. I need to keep things simple in the w/h. Adjustment without welding the old holes shut, cutting down the transom and dealing with resealing things will make testing easier. We shouldn't be adjusting underway once thing are working properly, hopefully.

Here are some new pics of the instalation that may help, it looks totally by the book.

Thanks again, Tom







Here's a couple shots of the mounting height using a strait edge on the bottom edge of the cav plate inboard and outboard of the stbd engine.

[Manie B]

Sorry to dissapoint you - and i dont know the what the other members think - but for this application these motors are now only level / in line with the bottom - this is now where they should be 30 mm DEEPER in the water. You are NOT on flat inland water. This pic shows it clearly.

[tkinak]

Manie, The disappointment would be going through another season without curing this problem. I appreciate you bearing with me as I air things out trying to get a handle on what the problem(s) are.

You can see from this pic that lowering the engines without jack plates would require some cutting and welding. Then if it doesn't work we have a very short transom, it's already short enough. I'm researching the various manual transom jacks available. I see they make them in different setbacks, 6", 8",10" and 12" I assume 6" will be fine as this is not a racing boat?

I measured various points around the tie rod and a transom jack will put it out beyond any obstructions. With the exception of one fuel line there should be no issues beside turning bolts, I hope.

So it's off to the net to find the best deal on a pair t-jacks. Cook manufacturing and TH Marine are running around $220 US.

Thanks everyone! Cross your fingers!

[Manie B]

Hi, i see what you are getting at, not much left to go, the last little bit and thats it, getting tight.

Please post web adresses / brochures for the stuff you want to buy and i will give you my 2 cents worth. I dont understand what you mean by the 6 inch.

please remember you need a fair distance between the motor and the back of the boat to allow the motor to turn AND tilt and not to foul up the cables / hoses / steering etc.

I dont think that the difference is racing or fishing, you have got BIG POWER there and that is my focus / concern, the final correction will be to get right prop to run the WOT rpm for that motor which is usualy around 5500 rpm trimmed down, the revs will climb as you trim up for neutral / light steering usually not above 6000 rpm, but first solve the cavitation problem.

Dont forget to mount a strong chain from the motor to the transom (inside a rubber hose) i had nightmares of my extension brackets braking and finding my 225 sinking off - fortunately it never happenned - trust comes from hours on the water - never sit back and say all's well

[tkinak]

I assume set back is the distance the engines are moved aft by installing the jack. They make them different sizes 6" or 8" deep etc.

Another little piece of info I forgot is that the outboards do very little to trim the boat. Lifting one engine does not help overcome a slight list. Once on step you can trim both up a little before they slip appearing to have little affect on trim yet it does yield a little speed increase. Lowering the motors should help here.

http://www.thmarine.com/product.cfm?PRID=19

http://cook-mfg.net/cart/index.php?m...9a01d63cfdffec

http://www.boatownersworld.com/CMC/m...nsom_jacks.htm

[Manie B]

Another little piece of info I forgot is that the outboards do very little to trim the boat. Lifting one engine does not help overcome a slight list. Once on step you can trim both up a little before they slip appearing to have little affect on trim yet it does yield a little speed increase. Lowering the motors should help here

This to me is another indication that the motors are too high, not enough bite. Outboards invariably do affect nose up or down to some degree on every boat that i have been on. Motors at the correct depth should trim quite far up before slipping, you should even be able to project a rooster tail of spray before slip occurs.

I like the 10 inch unit from cook, you will drill and fit it to the high mark, because your motors want to go down - no need to provide for up. When i look at your transom photo i just hope that your cables and pipes to the motor wont hassle against the transom.

The more i see the more i feel that you will hit the sweet spot when the entire cavitation plate is around 50 mm = 2 inches below the boat, these brackets can do that with very good accuracy, on calm water you should even be able to adjust for fuel economy
http://cook-mfg.net/PDFs/PL-65%20Owners%20Manual.pdf

[Manie B]

tk what have you decided

[tkinak]

Hi Manie,

I'm shopping for brackets. I think your thoughts and other real world information I've heard is probably correct and worth trying.

The only thing that puzzles me is that Yamaha does not distinguish any thing different when installing twin engines verses a single. The only dimension says even with the bottom of the hull. Pretty much ticks me off because it's adding cost to the vessel.

I'm leaning towards the 6" because we have a 2" pipe behind the engines that protects them. The uncle is a contractor and I suspected that some day it would be used on a job moving floats or something. You can see it in one of the pics of the engines. It's pivoted up. It's also very handy as a step while changing wheels.

I'll keep you posted.

Thanks to everyone!!!!!

TK