Transom Bracket - twin outboards

[lobo1]

Must be a few of you guys running twins off of a bracket??

My boat, new to me this spring, is a 78 23' CC totally rebuilt with an Armstrong bracket and twin 2008 Yamaha F-150s.

Sounds nice - looks nice .....runs like crap(in rough water).

The props get air, they slip, the rpms spike etc - whenever in a heavy chop.
I put hydrofoils on both motors and they are nice but I still have most of the problem in rough water - we have heavy chop mixed with our swells here in Michigan.

Today I had the motors lowered to the last hole (down) and will run it tomorrow to see if it is any help.

Anybody else have any issues like this? Anybody run a jack plate to lower the motors DEEP after getting out of the hole (in the chop) -

anybody have any suggestions at all? I have inquired several prop experts and they feel that all props will slip to a certain extent with this bracket/twin setup until I fix the underlying problem...

Thanks in advance -

Steve

[Old'sCool]

What is the underlying problem the "prop experts" warn you about??

[lobo1]

well - I have very limited experience in this arena and am open to any and all advice (to consider) but what I have been told by some prop people is that in choppy seas .............

"It sounds like you are probably pulling surface air into the props in the chop. This can be a common problem during certain wave conditions, and it more prevalent with bracket mounted motors or stepped hull designs that tend to pull air under and channel it back to the prop"......if that is the case, going to a 4 blade prop won't make a difference, it will slip the same way...."

just a few excerpts -

any thoughts?

[BA17]

I run twin F150's on a twin bracket as well and have no problems. I have fished the boat in some nasty weather and have never had the props slip at all. I was thinking it may be a prop issue, but sounds like it may be something else. What length are the shafts? Could the bracket be mounted too high? Could the bracket be mounted too low?

[lobo1]

Wow! BA17 Glad to see you chime in here!!!! maybe you and I could compare a little information.

This boat I bought is wayyyy too nice to not be able to enjoy the ride in rough water! As it stands now - I can only run about 18 mph in the slop w/o prop issues.

Let me see what I have tomorrow with the motors lowered to the last hole and then if need be - I could compare your specs to mine????

[peterb]

Been there & done that. For years I ran a 1972 23' with twin 200 HP OX66's (see below picture). Also, it had an enclosed transom and a full flotation bracket.

I have the following questions for you:

Does the boat sit stern heavy in the water?

Do you have any balast materials up front?

What type of props are you using?

What kind of trim tabs are you using, and are you sure that they are properly working?

Have you checked your bracket to see if she has any water leaking into her?


Here is what I did with my set-up to get her dialed in.

I put 200 pounds of lead up front in the bow hatch

I had the engines mounted in the mid level setting on the bracket.

When the engines were at the highest level setting my 3 blade 19 pitch & 15.25 sws props were capable of coming out the water. When I put on 21 x 14, 7/8 props they did not come out of the water anymore.

SeaCraft's with brackets and alot of engine weight need alot of trim tab. I had 16 x 12 Bennett sportman trim tabs. I was not happy with their performance initially. I pulled the boat of the water and saw that the tabs were not working that well. It ended up being that I was a little low in transmission fluid. When I filled the system up, I had far more responsive tabs and a better handling boat.

I eventually went to 4 blade power tech props. They gave me decent stern life and improved the boat handling. My speed at WOT went down from 56 MPH to about 47 MPH, but I was happy with the trade-off.

If you have any specific questions, please let me know.

Regards,

Peter

[doodlebug]

sounds more like a prop problem. I run my motors real high
on my bracket without breaking loose.get someone to run the boat and while running check and see where your cav plates are in the water with my 250's I had to run an inch and a half up above the water to get out of the hole. my 150's
run right on the surface also look to see if a transducer or somthing is making some problems.Also the gulf down her gets real sloppy.

[Fr. Frank]

I agree that you have a prop problem. But I'll also bet you need your whole setup tuned.

Do this test: With the motors trimmed fully down, with no trim tabs in use, while at about 4000 rpms, turn the boat sharply left and right, as sharply as you can. If you get the outside prop ventilating in the turn, you've also got a mounting height problem, for either the motors, or the bracket, or both. (This test assumes factory-cupped propellers.)

Let us know.

[lobo1]

lots of good info here. will print it out and post tonight or tomorrow morning with results on my trial from today.

i have added 90 pounds of lead to the front to see if it made any difference. I didnt notice too much..... I dont think it sits stern heavy but I will take a few pictures and get your opinion.....no water comes in the scuppers with me in the back sitting at the dock.

where the motors were mounted until today and with my last setup - my boat pretty much jumped on plane. I put the throttles down all the way and would maybe get 6-8" of bow lift......this is after adding the hydrofoils.

lots of variables here to work out but at least I have some guys here that have some experience with this very issue and I am optimistic it will work out.

thank you all

[Old'sCool]

Lobo, Post a couple of pictures from different distances of the stern kind of parallel to keel. I would like to see where the cav plates/tabs are in relation to everything.