Height of motor question.

titaniumneck

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I guess I'm on the right thread, but here I go. I have received all the input about my switching my 40 tohatsu from my aluminum boat to a 15 ft sprint fiberglass questions on propellers.
Now my question is, right now I have my motor on small jackplate that put the cavitation plate about an 1 inch above bottom of boat, about 6 inches back from Transom. Boat now will plain out with moving weight to front of boat until speed is up, then you can sit in riding seat. (no power tilt) 11 pitch aluminum prop with holes.
Is this the best position for my motor? I would love to get a bigger motor, but not an option for me. Should the motor be higher or lower, closer to Transom or further back?
Top speed right now is 27mph which is fine by me, but the moving around in the boat while taking off is not the safest thing to do.
 

Scott Danforth

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because you have a jack plate that moves the motor back, you can sit "higher" than normal. as far as is this the right height can only be determined by you trying other heights and going for a ride.
 

titaniumneck

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Unfortunately, it is a homemade jackplate from 3/8 thick c channel and it is not adjustable.
 

Scott Danforth

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then I say, if your happy with the performance, there you go
 

Sea Rider

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Can be happy with the water performance, but can still be bettered, but that isn't known yet. Post a pic of lower leg showing both horizontal plates sideways to have a look. Tohatsu OB's have their ideal own sweet OB/transom height if waning going per that tech parameter. From there can go for a prop maximization to pull all those 40 hores out as usually loaded.

Happy Boating
 

Sea Rider

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Reading post again, why does OB needs to sit in a jackplate in the first place. Remove jackplate, sit OB trimmed to 90?, go for a wot spin on flat water as usually loaded with deck weight well balanced, once on plae, pull head out transom and check where is water flow at speed is passing through lower leg. Will know if in need to lower or raise OB a bit.

Sweet OB-Transom Match.JPG

Ideal is for water flow at plane to skim right unde upper plate (white line) as in pic (1). If with water splashes towards or over transom, place wooden shims under swivel bracket till cond (1) is achieved.

If flow passes above red line can use the jackplate to lower OB till condition (1) is achieved. No way you would like to chop transom down to get rid of the jackplate. LOL!! Assume a jackplate unbalance more OB weight aft than sitting on transom which will need to be compensated shifting or adding more weight forward for opt combo balance.

Happy Boating
 

titaniumneck

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Also wow, I have been looking at the wrong plate forever, the x plate on your picture is the one I always thought has to be a inch higher than bottom of boat, now I see that you show its the plate above that one. So the plate with the white line should be a inch higher than bottom. Is this accurate?
 

ondarvr

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Also wow, I have been looking at the wrong plate forever, the x plate on your picture is the one I always thought has to be a inch higher than bottom of boat, now I see that you show its the plate above that one. So the plate with the white line should be a inch higher than bottom. Is this accurate?

​You are correct in how you have it mounted now, don't lower it.

​The exact hight can be different for every boat and motor combo, even the exact prop design comes into play, then add in how you use the boat, this results no "right" answer unless you do all the testing.

You can still raise the motor if needed, just drill new holes in the jackplate and move the motor up.
 

Sea Rider

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Forget all about if it has to be about 1 inch higher than rear keel, that's pure speculation, useless word of mouth going over and over. No OB brand has a fixed height, varies between 10 to 25 mm, boaters don' like reading Owner's Manuals, that's for sure. Still doesn't say much...

In real boating world need to rise or lower OB accordingly for water flow at speed to skim right under white line which is the small upper water deflector plate. OB must be trimmed at 90? as in posted pic. Need to determine height and what needs to be done or corrected.

Been working with Tohatsu and rebadged OB's for several years now, have optimized many boats including the last 4 of mine that way, Look at the performance chart achieved after much water trials at different lower leg heights. know what works spot on and what doesn't.

Lower Leg Water Performance.JPG

Seems you have a short shaft transom with a long shaft engine, right. Some transom are not short neither long, are medium. What's the distance from middle upper transom to lower middle keel in a straight line in centimeters ?

The exact lower leg height is different for every boat's hull and OB, can determine exact height by looking out transom once hull passes the transition from sitting down flat towards upwards when on plane.

Happy Boating
 

titaniumneck

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Well, I took the advice from here and downloaded the m40c tohatsu manual, and according to the manual, my motor is about 3 inches too high. The top lower unit plate should be level with the bottom of the boat, and the plate over the prop should be 1.5 inches below boat. Now it's about 1.5 inches above. Now I guess I should follow the manufacturers guidelines and lower it.
 

ondarvr

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Many of the recommended methods are for a very "safe" initial starting point for set up, it's how a dealer may mount the motor for an average first time owner that has no clue as to whether it's correct or not, it's not for best performance with the exact combination of variables I listed above. If you look into actual fine tuning of the motor height for best performance you will find that the motor will be mounted much high than what's in the pic. This is why you need to actually do the testing on your boat, don't rely on what someone else says "might be right".
 

Sea Rider

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Well, I took the advice from here and downloaded the m 40C tohatsu manual, and according to the manual, my motor is about 3 inches too high. The top lower unit plate should be level with the bottom of the boat, and the plate over the prop should be 1.5 inches below boat. Now it's about 1.5 inches above. Now I guess I should follow the manufacturers guidelines and lower it.

You are giving to many turns to the subject. You can follow those parameters as per manual, with that setting will not know at which lower leg height will water flow pass when combo is on plane while riding parallel to water level, look over transom and compare it to previous posted performance pic chart as to know if will see the light at the end of the tunnel shortly...

Small Upper Plate.jpg


This opt lower leg height water tested zillion times is what you need to dial for combo to run as a champ. If OB needs to sit higher or lower on transom for that to happen, it's your personal story....

Happy Boating
 

titaniumneck

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Well, I don't have to worry about it for a while. Motor stopped running about 7 miles from boat launch, I knew what it was, needle valve/on float was sticking. Well I always have tools on boat, so I let my son take over trolling motor and let him fish while I pulled off carburetor, easy enough. Well, my awesome self broke the bracket for the float pin, both sides popped right off. Running back to the launch wasn't too much of a problem as long as it was wot. But I quickly learned without needle valve to just pinch off the gas line when I wanted to idle. But now I gotta buy a new carburetor.
 

ondarvr

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I've been on this forum for long time, and I think this is the first time someone has broken a float mount on the water and then figured out how to make it run right so they could get home. Always a first, good job on getting back to the dock.
 

titaniumneck

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Thank you Mr. Ondarvr, it has been like this my whole life. That's why I always have tools. I can remember almost 35 years ago, while coming back from fishing my dad's Datsun trucks fuel pump went out, so he took the portable gas tank from boat, put it on floorboard of truck, mounted the line to trucks carburetor and I held my hand out the window pumping the gas bulb until we got home, from Ramah to central by baton rouge, about a 30 mile drive. Gotta think on the fly.
 

Sea Rider

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Securing fuel tank on roof would have been a much better option, open tank's air vent, connect hose end to carb, prime hose bulb till hard once, start the engine and let fuel flow down by gravity. LOL!!

Happy Boating
 
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