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Old 21 May 2013, 16:54   #1
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tohatsu 18 engine depth

Hi iv just bought a 18 ho tohatsu long shaft to go on my old 12 ft zodiac sib..when the engine is on the boat the cavitation plate on the engine is about 6' lower than the bottom of the boat....its go hydra foils fitted...iv run it for a few miles today and its great..doesn't poor water into the boat and jumps fairly instantly onto the plane......iv easy access to a tig welder and aluminum...is it worth the effert of building a transom extension to lift the engine upto the correct height?? Will it make a lot of difference..speed handling??
Thanks in advance for any info...
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Old 21 May 2013, 18:04   #2
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What's your transom height in cm ? Do you have inmaculate flat water flow, or slight water splash splasing back transom ? My long shaft 18 gave excessive water shower out transom.

Check this to see if we are speaking same Tohatsu language.

http://www.rib.net/forum/f36/matchin...som-55140.html

Happy Boating
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Old 21 May 2013, 19:03   #3
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From the bottom of the boat to the top of the transom is 16" or 40cm

The engine from the top of the engine mount were it sits on transom to cav plate is 55 cm 21 1/2"

I ran it today on perfectly flat narrow water ( bit like a canal). The water from the back of the boat wasn't splashing up like the pics in the link..
I'm goin to run the boat on rivers...7 and dee. And will be going in the sea.....
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Old 21 May 2013, 19:06   #4
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Old 21 May 2013, 19:12   #5
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Old 21 May 2013, 19:16   #6
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All as iv done is straight line stuff haven't a clue what its like in turns or deep water...its up on the plane within a couple of seconds with me steering 16 stone and my friend in the front 19 stone....I haven't run it without the hydro fins they are a real pain when trying to carry the motor to the boat on my own
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Old 22 May 2013, 03:57   #7
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woz,

The fins are a good debatable point! If you do a search here (try also "Doel fins") you wll find much heated debate on their pros & cons.....

One thing is certain - they give lift so will get you on the plane much quicker.

Down side if your engine is a long on a short shaft sized transom, those fins will create drag, as will the extra leg length. (Some peeps have posted pics / reports that their fins leave the water once on the plane - that certainly won't happen with excess leg length.) Might be worth trying without them. Worst case - you bolt them back on!

If it's running fine then maybe you don't need to raise it, BUT be aware that you have a lot more underwater, so be careful near beaches / shallow water etc!
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Old 22 May 2013, 06:42   #8
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And you also have 6 inches further to reach out should you need to clear the prop of plaggy bags or weed.
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Old 22 May 2013, 06:47   #9
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I'll fabricate a transom extension up and see if it makes any differece....I'll post some pics and let you know
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Old 22 May 2013, 08:08   #10
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Other thing to watch is weight.....

To raise an engine 6" I'm guessing a chunk or two of 1/2" ply might be involved.......
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Old 22 May 2013, 08:38   #11
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I'm not at the boat at mo but I think I willl have to raise the engine 100/120 mm so ill make a sandwich bracket out of alloy..should only take an hour with the tig welder n the weight should be minimal.....it says on the transom plate it can take an engine upto 40 so the extra leverage caused by raising the engine should still be ok.....
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Old 22 May 2013, 09:43   #12
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You should be experimenting some sort of small splash out transom, bear in mind that water flow is hitting the round portion of lower leg at speed and not the edge portion where water flow cuts as cutting a cheese.

With mate sitting at bow, with perpendicular engine sitin on transom go for a wot spin, when on plane pull your head out transom and check water flow passing through lower tail, if back splash out is minimum leave it there.

Doel fin must ride paralell to water level when sib is on plane, if not, will create extra water drag there. Besides a pain in the but when placing, retrieving engine from transom, could turn into real garbagge collector, collecting kelp, plastic bags, banana peels, you name it when boat is on it's way.

If wanting engine/transom height boating perfection match make a SS 5" engine mount..

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Old 22 May 2013, 11:23   #13
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Tried this with 2 x 1/2" ply and bolted it in four places 2 down the extension and 2 on the transom as I fabricated beach wheels running channel Ali the length of the extension and half way down the transom. On my 9.8 Tohatsu long shaft it cracked the ply on first attempt unbelievable amount of power for such a little engine.

As suggested make it from metal.

Apart from the splash back having the engine lower actually resulting in me being able to keep the throttle open more in my little <3m SIB and I kept up better with the bigger boys!

Would be interested to see the finished transom extender
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Old 22 May 2013, 13:39   #14
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I'll try and get it done for next week..will make it from 2mm stainless steel or 4mm alloy..not sure what's knocking about in the workshop...have scetched it out so just need to make it now...ill post pics of it made and then fitted...
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Old 22 May 2013, 14:00   #15
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Quote:
Originally Posted by woz72 View Post
I'll try and get it done for next week..will make it from 2mm stainless steel or 4mm alloy..not sure what's knocking about in the workshop...have scetched it out so just need to make it now...ill post pics of it made and then fitted...
If you make it out of stainless make sure that you put some wood for the engine saddle to bed into ,
Once saw a brand new engine twist itself off when the guy put full lock on then have it loads of power
the clamps even though they were tight just slid .
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Old 22 May 2013, 16:08   #16
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Iv been upto the workshop and its low friction stainless or aluminum so its going to be alloy...iv got a 30 mm piece large enough to make the spacer and some 4mm sheet to make the outer shape...ill get some pics up as soon as I get it fabbed up....thanks for all your input chaps you have all been a great help..
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Old 23 May 2013, 02:51   #17
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If yopui are fabbing it yourself, whty not weld eitrher a lump of wire or jsut build up a "circle of weld", which will at least stop it going over the side...

I've had a 4 slide on an ally plate.


I do have to ask. Would it not be easier / neater / lighter to trade the longhsaft in for a short?
(I relaise it's the engine you have etc etc.)
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Old 23 May 2013, 04:07   #18
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Hi....this engine is a 2004 2 stroke used once a year to push a fishing boat...its hardly been used..never seen salt water never been opened up and came for the right price...the mount should look ok..ill use the original piece of cast alloy off the transom to go onto the extension so the motor mount has something to grip too..my boats a 1971 zodiac so it doesn't look the best in the world anyway....if we use it this season and take to it I'll get something better for next year....there a 3.4 m. Zap cat on eBay at the moment lol
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Old 23 May 2013, 10:05   #19
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If you already have a 16" transom height, a good start will be to make a engine mount with at least 4" to match engine's 20". That's in plain theory, in practice under trial & error will probably need to raise 0.5" or bit more.

Sit engine on transom at a height (?) so to achieve a undistubed water floww passing slightly under small upper deflector plate, that's the sweet heigh position for that engine.

SS or alum plates are slipperry materials, will be good to glue, vulcanize 2 rubber, pvc bands or whatever material suits on both sides of engine mount, front engine will rest much confy on rear mount and engine clamps will lock and avoid engine vibration much better...

Happy Boating
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Old 23 May 2013, 18:10   #20
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I'm fabbing the mount up at 120mm...iv some convayor belt which is 2mm thick so I thought I'd maybe use some of that bonded to the new mount..I'm aiming to do away with the hydro foils because it make moving and mounting the engine a real pain..iv a few jobs to get done tommorow then I'm hoping to sort the mount in the afternoon...
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