Anti-Cavitation Plate- Tohatsu 20hp

[Irish Boat]

Hi There,

I hope someone here can help me. I have a new Tohastu 20hp fixed on a Zodiac Grand Raid Mk III. My problem is that when the boat it at more than 1/2 throttle, it revvs up and it sounds like the prop is spinning. I tried cleaning out the fuel line etc and it doesn't seem to fix the problem. Upon reading the manual last night, it tells me to ensure that the anti-ventilation plate is below the surface of the water when running with the throttle wide open. It is not- it is about an inch above the water. Could this be the cause of my problem? I see on other threads about other brands that they reccomend the plate is just on the surface, or just above it, but never below. Why is the Tohatsu different? I presume the same is said about Nissan Engines as they are the same engines.

I hope someone can help. Thanks for taking the time to read this, and thanks for any replies.

 

[Sea Rider]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Welcome to the forums,

That's weird, that model uses a short shaft, what's your transom/keel height in centimeters ? Are tubes and keel well inflated, with right trim angle and correct passenger distribution ?

Happy Boating

 

[JB]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Howdy, Irish Boat.

Welcome to iboats.

You are most likely ventilating the prop. There should be no air under the AV plate at all. Though I prefer it to be right on top of the water, slightly below is safer than above.

 

[Irish Boat]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Welcome to the forums,

That's weird, that model uses a short shaft, what's your transom/keel height in centimeters ? Are tubes and keel well inflated, with right trim angle and correct passenger distribution ?

Happy Boating

Thanks for the reply Sea Rider,

It is an old GR, late 80s i think, so the transom height might be different. Now that you mention it, i have a leak in the keel, and the issue doesn't seem to be there when I set off. It seems to appear when the air has leaked out. I'll go to the pier and measure the transom etc and get back to you. thanks!

 

[Irish Boat]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Howdy, Irish Boat.

Welcome to iboats.

You are most likely ventilating the prop. There should be no air under the AV plate at all. Though I prefer it to be right on top of the water, slightly below is safer than above.

Thanks for the reply. I think you are right. Is there any other solution other than cutting the transom down?

 

[kfa4303]

Re: Anti-Cavitation Plate- Tohatsu 20hp

you could make a jack plate, or lowering plate as it were using a few pieces of aluminum angle and some stainless steel hardware. Do a search here in the forums for lots of ideas, of course you'll need to invert the design to allow you to lower the motor, but should otherwise be the same.

 

[Irish Boat]

Re: Anti-Cavitation Plate- Tohatsu 20hp

you could make a jack plate, or lowering plate as it were using a few pieces of aluminum angle and some stainless steel hardware. Do a search here in the forums for lots of ideas, of course you'll need to invert the design to allow you to lower the motor, but should otherwise be the same.

Thanks for the idea. I'll measure the transom first to see if it is the correct height.

 

[pvanv]

Re: Anti-Cavitation Plate- Tohatsu 20hp

If the transom is taller than 17", you need a long shaft motor. The anti ventilation plate needs to be at or slightly below the surface at speed, or the prop on any motor of any brand will suck air. You can also burn out the impeller and overheat due to drawing air instead of water at the water inlet.

 

[Irish Boat]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Thanks guys,

Measured the transom and it is under 16ins. Could the leaky keel tube be the problem? Or am i looking at something mechanical?

 

[Sea Rider]

Re: Anti-Cavitation Plate- Tohatsu 20hp

If got you correct, water flow is passing under or slight over AC Plate ? Check pic. On 2 stroke Tohatsu engines mounted on transom near or 16" heights would be ideal for water flow to pas slightly under small upper plate when at wot. Assume both 2 and 4 strokes engines have 3.5" small upper plate to AC Plate heights. Have said nothing if you have tested with well inflated sib with minimum 3.0 psi on all tubes including keel + well trimed engine and correct passenger distribution for top performance.

Don't know if you are test guessing right engine trim; place boat on water and trim so to achieve a perpendicular engine and/or paralell AC Plate with respect to sea level. Move one passenger at bow, full throttle and check if with prop venting issues. Keel has nothing to do, seems just a transom height issue. To chop down a transom is not rocket science, probably will need to chop down at least 2". A boat shop can do the work for you.

Happy Boating

 

[Irish Boat]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Thanks for the detailed response. The water is nowhere near the upper small plate at WOT. My problem is that i am usually alone in the boat, so it it difficult to get the balance right. I'll go out this weekend and try some different balancing tricks. Only other idea is to cut the transom.

 

[Sea Rider]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Detailled response is the only wat to get the right idea on spot, we have learned the hard way with exhaustive trial and error sib/rib engine heights experimentation. Start inflating sib well as to have a rigid transom on tubes, trim well engine, place a mate, gas tank at bow, go for a spin and check if with prop issues at plane. Is with slight better performnace than before, you are confirming a transom chop is mandatory.

Happy Boating

 

[kfa4303]

Re: Anti-Cavitation Plate- Tohatsu 20hp

HI Irish. Cutting the transom should be your last resort. Try moving forward a bit using a tiller extension. I made mine out of 1 1/4" thin-walled PVC and couple pipe clamps for about $5 and it works beautifully. I can can now get on plane much more quickly and stand up while underway to see where I'm going, which is very important here in Florida where we have lots of very shallow water (<12") and other obstacles like sand and oyster bars, sharks alligators, manatees, tourists to avoid (well, except the tourists. I try hit as many of them as possible ). I also added a PVC grab bar to hold on to while underway. Not only does it provide stability, but it has allowed me to use a lot more vertical space in the boat to mount accessories. Between the two mods, it's like I have a whole new boat for less than $50 total! Here are some pics of my set up and some vids of guy using their extenders.







http://www.youtube.com/watch?v=5zC7cEuByNs

http://www.microskiff.com/cgi-bin/yabb2/YaBB.pl?num=1299183905/45

http://www.youtube.com/watch?v=sTR4m5z46U0

http://www.youtube.com/watch?v=cZMR97duw_Q

 

[Irish Boat]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Thanks for the input- your setup reminds me of one of the boats on Swamp Men! Time to experiment i think

 

[kfa4303]

Re: Anti-Cavitation Plate- Tohatsu 20hp

oh yeah I just got back from a trip down the swamp/river today. Saw some manatees, osprey, crabs, fish and some other cool stuff with my little nephew. The grab bar makes all the difference in the world! Not only can you run at any speed from idle to WOT while standing to get a better view, but when you're anchored up the bar makes a great t-shirt/towel rack to help keep everything high and dry

 

[Sea Rider]

Re: Anti-Cavitation Plate- Tohatsu 20hp

You cannot compare the ride of a hard hull & keel boat to an inflatable, worst if under inflated, he'll need to determine by triming & ballancing well his sib if transom is way high for his set up. Placing 1 or 2 mates at bow will be same as using a tiller extension to move driver a bit forward when boating solo.

Happy Boating

 

[Irish Boat]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Just spent the day spinning around and i can report the following:

First hour, fully inflated keel, balanced weight, me sitting halfway up the tube and other stuff up in the bow: no problem.

Second hour, keel begins deflating, but some water in boat (due to hauling pots) problem reappears but not too bad.

Third hour, trip with pots onboard and some water, no problem at all, perfect in fact.

I think that the boat was too light with just me and a deflated keel, and the Atlantic swell was raising the stern out of the water too often. I dint think i need to go cutting, just balance the boat better.

I want to really thank everyone wo took the time to help me. This forum is fantastic.

 

[Sea Rider]

Re: Anti-Cavitation Plate- Tohatsu 20hp

Nice to read it's working better than before, check that keel, should not lose pressure, When thinking having engine/transom height issues the ideal would be experimenting at wot on flat, calm no windy water courses.

Happy Boating