Trim tips

[jondavies]

I have an Avon 4.6M SuperSport with an aluminum floor and a Tohatsu M40C. After six months of headaches, I FINALLY got the motor running properly and took it out for its first long-range outing last weekend.

The problem I'm having is with the trim. The boat gets up on plane and runs for about 15-30 seconds but as soon as it goes over a wave of any size, the prop aerates and the motor over-revs. I had to feather the throttle and keep the speed at 12 mph or less to reduce the chances of this happening.

Occasionally -- and without any changes to the configuration of the boat -- the boat will come off plane and get bogged down. When that happened, I could run the motor WOT and the boat would max out at about 10 mph with lots of spray coming up from the bow and the bow flexing upward. If I stopped the boat and opened the throttle again, it would get up on plane and run fine until the prop aerated.

I tried adjusting the trim on the motor. With the motor trimmed all the way it would bury the bow and not plane at all. The more the motor was trimmed out, the more the prop would catch air.

Here's some background on how the boat was set up:

Transom -- cut down by previous owner to 15" and motor is a short shaft. At my boat dealer's recommendation, I put a shim under the motor so that the cavitation plate is just about 1" below the keel.

Smart tabs -- currently in the #2 position (#1 position being the least lift).

Load -- fairly heavy: two people + 4 scuba tanks + scuba gear + 18 gallons fuel + anchor + typical boating accessories. Initially, the fuel and scuba tanks were up in the bow with my son in the middle and me in the back. Later I shifted a couple of tanks further aft in an effort to keep the stern in the water but that didn't seem to make much difference.

I'm hoping someone can give me some suggestions on how to trim the boat out to make it run more efficiently. Options include:

Changing Smart Tabs to #1 position (least lift)

Removing shim from motor and running with cavitation plate about 2.5-3" below keel.

Altering the load distribution (more weight aft?)

Changing the prop (I'm not sure what pitch it is now, but the dealer put on one that he recommended for diving/heavy loads).

Thanks in advance,

Jon

 

[JimXeod]

Re: Trim tips

Jon,
The first thing I would recommend it to remove the shim and test it out. It sounds like you are just a hair too high on the motor.
Yes, I know everyone suggests within an inch of the bottom of the hull, but if it's just "slightly" deeper, it will assist with heavy loads. A lot of inflatables do not drop down in the aft end when loading up. This way you can trim it out properly. I had a similar problem last year, took out the shim, smooth sailing. It LOOKS like my cavitation plate is an inch too low, but by trimming "out" to the 3rd hole on the outboard it works perfect. Planes and trims out perfect, top speed awesome, and no aeration issues.

Give it a try. I bet it helps. Leave the prop pitch alone until you get this situation corrected or it will be a waste of money. Let me know how it works out!
Jim

 

[jondavies]

Re: Trim tips

Thanks Jim -- I'll try it and let you know what happens.

Cheers,

Jon

 

[JimXeod]

Re: Trim tips

I can almost assure you it will fix the issue. When I had my motor to where it was within an inch of the bottom it would catch air and overspin all the time, and I could not trim the motor out more that two holes on the transom clamp. It looked the way it was supposed to, but did not operate efficiently.
At present it looks about an inch lower than ideal, but performs flawlessly. I know that sounds strange, but it works. There is really no additional drag, because it is trimmed so that the bow is raised just slightly, and when on plane, the plate rides perfectly near the surface as designed.

 

[jondavies]

Re: Trim tips

I checked the boat with a ruler and the cavitation plate is 1.5" below the keel with it's current trim (in the middle hole) and the shim is 1.5" thick. If I take it out, the plate will be 3" below the keel. That seems a bit much according to the textbooks but, as you say, how the boat performs in the water is the only issue that matters.

Thanks again for the advice!

Cheers,

Jon

 

[Dry Diver]

Re: Trim tips

Could soft tubes cause the same type of problems? I am having a similar issue with my boat. I should have more than enough motor to get the thing up on plane with me and one other diver. It will do ok in flat, but as soon as the water gets choppy at all forget about it. I have a 14' SIB with a 40HP Merc.

 

[JimXeod]

Re: Trim tips

Yes, underinflation can cause this as well. Check the tubes with a pressure gauge.

As to the original poster. Just take the shim out and try it. Don't worry about measuring with a ruler! Inflatables don't always behave "textbook"!

 

[Drowned Rat]

Re: Trim tips

You for sure need to move the Smart Tabs to the #1 postition, and make sure you are using the 20 lb. actuators. Next, you may have too much prop for the weight you're carrying. Try going down on the pitch a little. Also, move the tilt pin all the way in. After you get the thing to stop aerating, you can maybe move that out a hole or two.

Underinflation can cause aeration and cavitation, but I'm guessing your Smart Tabs are the culprit. I would not lower the engine any more than it already is or you'll start experiencing other undesireable side effects. Last effort is to install a Doel-Fin which often times will fix an aeration problem on an inflatable that nothing else will.

 

[dazk14]

Re: Trim tips

Are you using a cupped prop? It solves all sorts of ventilation woes.

You can have your's cupped if the rpm are ok.

 

[Dry Diver]

Re: Trim tips

well I was out today and had more trouble with prop cavitation.

I inflated the tubes before I got in the water. Motored out about three miles, dropped anchor and went for my dive. 1 hour later when I got back out of the water, I again refilled the tubes (air temp was around 80, water temp is about 55-60) I figured that they are getting soft after being in the water for a bit. On the way back in I could not get it to plane out with me and one other guy in the boat with dive gear.

I don't think that I should be having this problem with the amount of motor that I have. The last time this happened to me on my last inflatable (Achilles 16') I cut about 1" off the top of the transom and the problem was gone.......Never came back........I think that it is time to get the saw out again........unless anyone else can help me out here..........

 

[JimXeod]

Re: Trim tips

Drowned rat is right. However, if you altered the factory transom to make it "look" right, it is probably having a negative effect. That is all I am saying. It is the easiest adjustment, and is free! Also, check the smart tabs as referred to by Drowned rat, maybe they are causing too much stern lift.

Dry diver, you could always get out the saw, but have you thought of a Doel-fin? Like drowned rat said, that may work. You have a lot of motor on the boat, and it is pulling in air for sure. I personally don't use them, but I know they work for many! After that look at your prop pitch. In my experience inflatables work best with the motor a little lower in the water than even with the keel for ocean use. In rougher water the massive bouyancy of the inflatable can cause prop aeration. Remember, these are not like a hard hull. They don't "slice" the water very well when it get's rough to create an un-aerated area for the prop to operate in. They perform different esspecially in chop, and open seas as compared to flat water.

What works in a lake, will not be ideal when you hit some rough stuff. The lift of the tubes makes them great in rough water, but you don't want the motor too high. It will keep falling off plane if the motor is slightly too high. Not blowing my horn here, but I have a lot of experience with inflatables in all types of seas, Military and civilian.

 

[dazk14]

Re: Trim tips

I'll repeat the obvious. Prop cupping!

 

[Drowned Rat]

Re: Trim tips

Yeah, I'm not a Doel-Fin fan either, except for inflatables under certain circumstances like this. They are essentially useless for providing lift or stability or pretty much anything else they claim to do, but they do one thing really well on inflatable boats. They break up the vortex of air that is created by the propeller from too much power too close to the surface. And since they are trying to provide some lift, they push a column of water downwards right over the prop. That's a good thing if you're problem is aeration.

I would still re-check your Smart Tab set up before doing anything though.

Good luck.

 

[Dry Diver]

Re: Trim tips

I am not running smart tabs on my boat. I was told that I would not need them because of the size of the motor that I am running. I in flat calm I have no problem getting up on plane. Only when the prop starts to cav. I will look into the dolphin.

Thanks

 

[JimXeod]

Re: Trim tips

I'll repeat the obvious. Prop cupping!

That should in no way be needed on a smalll inflatable with a 40hp motor.

 

[JimXeod]

Re: Trim tips

I am not running smart tabs on my boat. I was told that I would not need them because of the size of the motor that I am running. I in flat calm I have no problem getting up on plane. Only when the prop starts to cav. I will look into the dolphin.

Thanks

I think that is a better option than cutting the transom. I think it will work out for you.

 

[jondavies]

Re: Trim tips

Hi All:

Thanks for all the suggestions and discussion. Here's an update from today's sea trials after shifting the trim tabs to position #1 (least lift) and lowering the motor 1".

On the way out, the boat would plane and I could get up to about 15 mph (compared to 12 mph on the last trip) before the prop would aerate. Also, I noticed quite a bit more "backwash" on the transom, with some of the spray kicking up over the transom into the boat.

After doing a dive and some fishing with the fly rod, I headed back to the harbor and decided to play with the weight distribution. I moved two steel tanks and the anchor box (7kg hook plus 25 feet of chain) to the rear of the boat near the transom. With this gear and my 220lbs in the back of the boat I finally managed to get it to plane without aerating. I was flying over 2-3' swells at 25mph+ (can't say for sure because I was bouncing too much to read the GPS accurately )

A couple of footnotes: I already have a Doel-Fin on the motor. One person I spoke to suggested removing the fin as that might be providing lift. However, I thought the fin is designed to force water down onto the prop as Drowned Rat says.

I have the 30lb actuators on the Smart Tabs. I had spoken to Nauticus before purchasing and they recommended these for my boat but I'll see if I can exchange them for a 20lb set.

Thanks again for all the suggestions. I'll keep you posted on the continued progress.

Cheers,

Jon

 

[Howard Sterndrive]

Re: Trim tips

I'll repeat the obvious. Prop cupping!

+1

That should in no way be needed on a smalll inflatable with a 40hp motor.

there's no type of boat that will benefit more from prop cupping than an inflatable

Get rid of the fin, and the shim, get the prop cupped and it'll be a new boat.

 

[JimXeod]

Re: Trim tips

I agree with getting rid of the fin and shim. I do beleive prop cupping would help. My point is, it should not be this big of an issue without a cupped prop. I have set-up many inflatables, and did not use a cupped prop.
That is not to say it wont help, bit it should not HAVE to be done.

 

[Howard Sterndrive]

Re: Trim tips

My experience with prop cupping is limited to one success story, and I haven't been to RIBnet lately, but I remember seeing a lot of posts over there that basically go (paraphrasing):

"I cupped my prop, no more ventilation issues"

but my real world experience with ribs is quite limited.