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Old 27 July 2017, 10:45   #1
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tohatsu 20 mounting problem?

A few odd handling problems last weekend which I suspect may be due to the way my Tohatsu 20HP 4stroke is mounted. The engine is clamped to a sandwich of steel plate (inside the transom), the boat transom and a piece of 2cm thick marine ply. The ply is only 10cm deep and does not extend to the bottom of the the engine clamp arms - the thrust pin in particular is below the bottom of the ply plate. Is this the problem I think it is? I think it may allow the engine to twist forward as torque is applied when accelerating. Various unpleasant side effects including attempted submarining just as the boat is coming on the plane, and a torrent of water over the stern at the top of the displacement speed range... Is my thinking right and do I need to fit a bigger ply plate or could I just shim in a piece of ply to support the bit of the clamp not currently supported?
Hope the picture helps explain my issue.. Any thoughts / comments welcome.
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Old 27 July 2017, 11:43   #2
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If your transom is wood then it looks quite firmly bolted. However I would get a larger bit of marine ply to replace the current piece (not to add a bit on) so the whole of the OB contact area is supported.

What's on the inside?
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Old 28 July 2017, 12:21   #3
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The inside has a steel plate which supports the clamps and the four through bolts. Now looking to source a piece of 20mm marine ply (or possibly nylon?)
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Old 28 July 2017, 12:52   #4
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Ok, that does look a bit odd. Who set it up?

My transom is wooden - just varnished (where yours is painted) probably 1" thick. I don't have any wooden patch on the o/side of the transom the engine is just bolted and clamped directly on.

I started off with my Longshaft as low as it would go, toggle clamped and bolted (only two bolts on my design). I moved it up a few years ago in the hope of more speed but didn't notice a change.

I'm not near the boat but will get you a pic sometime.

You are quite high trimmed already but submarineing sounds like stuffing to me and I have only done that properly once. The error there was definitely in front of the transom not behind!

You'll have more weight aft than I do (prob heavier engine + I have a console and my not insignificant weight at it), but if anything that should bring the bow down for me.
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Old 28 July 2017, 13:48   #5
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Stevie as poly says put your engine directly to the transom and bolt through with large SS penny washers unless your bolts are too long then have a plate on the inside HDPE would do the job from direct plastics they will cut to size and send the waste to you, make sure your cavitation plate is level with the keel or just above or below but put a shim in if there's a gap between transom and engine. (Just above or below you might not have a choice)
I did notice on the last day you were porposing the bow lifts then drops down slowly that's ok until you hit some choppy water then you stuff the bow. The reason is you have it trimmed too high and your lifting the bow but the engine cannot maintain the lift hence the term porposing.
The ply you have in is causing stress on the engine and transom because it's too short but I would get rid off that side.
The other thing is your boat heals on your side so you need to componsate for it Sib'S are a bit kinder that an hard bottom boat the problem with it healing to one side makes for a difficult attack to an oncoming wave and worse in a following sea a level trim is really important I've not seen polys boat but he has a centre console which helps a lot with the symptoms you have and weight distribution. Hope that's constructive
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Old 28 July 2017, 14:41   #6
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Jeff my hull is the same, but as you say the fat guy is in the middle.

The porpoising is interesting, I wonder if as the bow lifts either the helm twists the throttle (inadvertently) or even backs off a little contributing to the porpoising. A bit of the knack is having the bottle to keep the power on. If you are sitting at the side and about to get a free shower that might be more likely too (a gecko helmet makes a big difference to the effect of clipping a wave). Fuel, anchor etc should help balance the boat port/stbd.
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Old 29 July 2017, 05:32   #7
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Quote:
Originally Posted by Poly View Post
Jeff my hull is the same, but as you say the fat guy is in the middle.

The porpoising is interesting, I wonder if as the bow lifts either the helm twists the throttle (inadvertently) or even backs off a little contributing to the porpoising. A bit of the knack is having the bottle to keep the power on. If you are sitting at the side and about to get a free shower that might be more likely too (a gecko helmet makes a big difference to the effect of clipping a wave). Fuel, anchor etc should help balance the boat port/stbd.
I agree weight distribution is key easy when you have crew,
the porposing I know from my rib days with power trim and tilt too far up and it would start looking at my tilt gauge I could see instantly as ideal tilt was quarter on the gauge, I did notice stevies trim well up possibly the last hole so the bow was well up underway, on the flat the boat goes really well I think if he gets the engine flat to the transom to get rid of the leaver effect that ply is causing weather he desides to put a plate each side to protect the transom that's fine then get the height right that might be a few tests to see which is best for that boat before bolting up perminant then shift weight to balance his sat on one side he might have to bite the bullet and carry a bag of sand when single crewing.
Re getting wet that might just be a getting use to the boat and how it tackles the sea using the throttle more to lift the bow or dropping the revs with a following sea before you hit the wave in front his boat will surf well if the wave behind catches up I think it's just little things that's made life a bit uncomfortable in the rough stuff other than that he had a whale of a time it's a great boat and suits his needs in hinesight we should have jumped in with him to get the weight right then go from there

+1 for the gecko I have the gath and used it spot on
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Old 29 July 2017, 12:04   #8
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Oh how wet you get definitely depends how experienced you are on the throttle. I have the advantage of being able to stand up too, but if son is helming then it is definitely wetter!
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Old 31 July 2017, 17:14   #9
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let me know if you get sorted out with a piece of Ply...... my mate been doing up a little speed boat and has some 18mm marine Ply left. could post you up a piece? 12" square? Would be next week as away at the moment.
Cheers Al.
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Old 31 July 2017, 23:25   #10
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Thanks for the advice guys, much appreciated. I will be replacing the short ply with a piece of 20mm nylon (commercial kitchen chopping board). Will avoid future delamination/rot. So thanks Al, but I think I'm OK. For the record, I normally sit central akimbo the rear thwart. I can steer by bodyweight adjustment only. I only sit in the rear quarters for close quarters stuff or when baling... I'll do the switch in the next week or so and report back.
I think the outboard leg is pushing a long way forward. Think engine trimmed all the way in and then some. Gives lift to the rear and very quick on the plane but the cavitation plate creates drag area and transom flooding at high displacement speeds and going onto the plane it tends to bow down until forward resistance eases when the engine swings back to where it seems to be trimmed... I'll try the new backing before playing with the height. The transom seems to be right for a 20inch long shaft. Thanks again for the thoughts.
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