Jack Plates

pow2gran · 14 January 2013, 22:30

Having read about jack plates in PBR I was wondering whether one would improve the flow of water around my Tohatsu 60hp. I suffer from excessive water splashing up onto the rear of the tubes and also onto the engine cowl. The following photos were taken a little while ago. I have fitted a stainless prop which has made a bit of an improvement. I have tried all the different heights the engine can be set on the transom. None have made any real difference. I could imagine a jack plate that set the engine back by 4 inches and put the prop in cleaner water would work. I would appreciate any comments

Chris · 14 January 2013, 22:58

Quote:

Originally Posted by pow2gran

Having read about jack plates in PBR I was wondering whether one would improve the flow of water around my Tohatsu 60hp. I suffer from excessive water splashing up onto the rear of the tubes and also onto the engine cowl. The following photos were taken a little while ago. I have fitted a stainless prop which has made a bit of an improvement. I have tried all the different heights the engine can be set on the transom. None have made any real difference. I could imagine a jack plate that set the engine back by 4 inches and put the prop in cleaner water would work. I would appreciate any comments

When you have the engine trimed out does it still do this?

pow2gran · 14 January 2013, 23:15

I have tried most permutations and all trim levels.

Chris · 14 January 2013, 23:16

When your engine is trimmed fully down (in) where does the plate above the prop sit in relation to the bottom of the hull?

Locozodiac · 15 January 2013, 14:04

Probably you have a long shaft engine sitting on a short shaft transom, looking at both pics engine sits too low, water flow hits the upper small plate, where the splash is produced. It's not a prop issue, its' rather an engine/tail height issues.

What you should do is make a large metal plate to sit on transom so to have a stright transom. Would be faster than filling the middle transom cut with wood, resin, fiberglasss ? Check pics for understandig. Water flow must pass under the small water deflector plate, this is the sweet spot, gives best engine top speed, superb performance & impecable tight turns at any water conditions.

This is what you should aim to achieve once engine sits well on transom :

Tohatsu 20hp w/ 3/4" shim and proper trim - YouTube

Happy Boating

Turbodiesel · 16 January 2013, 20:23

Yeah, looks just like its to deep in the water. But you said on the phone you have tried raising the engine height?

Bushrider · 16 January 2013, 21:00

Quote:

Originally Posted by Locozodiac

Water flow must pass under the small water deflector plate, this is the sweet spot, gives best engine top speed, superb performance & impecable tight turns at any water conditions.

Looks strange and too low to me .. my best results I achieve when the A-cavitation plate is slightly above the water surface when the boat is on plane ..

pow2gran · 16 January 2013, 21:59

The engine has been set up as it says in the manual as shown in the picture below. I have taken the engine up two holes on the transom which is 17.5mm increments each time. It did not make any real difference but when it was two notches higher the prop cavitated when turning at speed.

I thought that the use of a jack plate would allow the prop to run in cleaner water. I would also be able to raise and lower the height to see if I could optimise the position of the motor in relation to the transom.

I have also fitted dolfins but that also didn't make a difference.

Something I feel is not right. Is it just the Tohatsu and would another outboard perform in a similar way? A jack plate is obviously cheaper than changing the outboard.

simsy · 16 January 2013, 22:50

How far is the bottom of the anti ventilation plate/dolphins above or below the keel line on your setup now?

Jack plates can sometimes compromise the tilting of the motor out of the water... As they can hit the transom when fully out...

pow2gran · 16 January 2013, 23:04

The ventilation plate sits around 20mm below the keel.

I see what you are saying about when on tilt, but I think it will clear the transom

Locozodiac · 17 January 2013, 01:18

Tohatsu gives worst spalsh back issues compared to other brands engines like Yam, it's a tail design, what stated on owners manual is just a refference from where to start but can vary depending on the boat's hull shape.

The only way to know how high you need to raise the transom is go for a wot spin with well ballanced boat as to have boat near paralell to water level and have a light spotter check how high water flow is passing above small upper plate, that's the distance you need to raise transom for engine to perform as expected.

The tech explanation is very simple, under upper deflector plate there's a triangular area that cuts perfectly water flow, over small plate, where the muffler is, the shape is squared, no water flow is being cut, that's the horrible splash area.

My rib/Tohatsu engine had same problem now that engine has been perfectly height tunned the overall handling, speed and performance is oustanding.

In this example the 30 HP engine was sitting 1" lower, was impossible to plane, excessive water splashed over transom. Wood shimmed the middle cut to have a straight transom and problem was solved forever. This fine tunning height experimentation must be done under trial and error and above all, boater must have a bullet proof patience to do it once correctly and forever..

Happy Boating

Locozodiac · 17 January 2013, 15:06

Some issues :

You are runing a rib, right ? Ribs usually have straight transoms, was that rib bough brand new or second hand ? if second hand probably previous owner must have chopped center transom down to use it with a short shaft engine? Your 60 HP Tohatsu is a long shaft engine.

Can you post your actual transom keel A-B ; C-D distances, check pic.

To solve your splash issues, you have 2 options ;

1-Find or make a well rigid built transom mount to bolt in middle transom so to raise engine. Tohatsu 60 HP weights 108 KG if being 2 stroker, more if being a 4.

1-Close middle cut with wood shim and fiberglass it to have a stright transom, make a wide and long stainles steel inverted U plate so to have inner and outter engine pads, bolt it to transom and go from there, water flow should pass well under upper small deflector plate eleiminating forever water back splalsh.

Happy Boating

pow2gran · 17 January 2013, 20:15

Locozodiac

Sorry but my boat is garaged at the moment so can't measure the transom.

The boat was new from Menai Marine and is the 480SR and is supposedly matched to the Tohatsu 70hp.

Thanks for the info it is much appreciated. I get what you say and appreciate yo pointing out where the water should flow.

Nick

Locozodiac · 17 January 2013, 20:49

Nick,

As an example, if you sit same Yam HP at transom, will have no or less water splash than Tohatsu, mostly because both lower tails shapes are completely different. Manufacturers in theory builds transoms to match every engine brand, but in practice on water it's a different story. That's why you need to fine tune engine tail height, preferably on flat no wind waters.

Happy Boating

simsy · 17 January 2013, 21:36

[QUOTE=pow2gran;511258]The ventilation plate sits around 20mm below the keel.

It's pretty much at a standard position, so matched as a starting point correctly.

The difference in shaft length from my 30hp from short shaft to long is 148mm, a lot.

My ventilation plate was 62mm above he keel line and it ventilated 5 out of 6 times when attempting to get on plane, with motor trimmed fully down, no joy at all level or trimmed up...