Cruisers & Sailing Forums (
-   Auxiliary Equipment & Dinghy (
-   -   Dinghy Prop Question (

The Mons 21-09-2008 12:29

Dinghy Prop Question

We just received our AB 8AL dinghy. This is an eight foot plus, aluminum hulled RIB. We attached a Yamaha 4 hp 4 stroke engine to it. My wife, I, and the dog weigh about 425 pounds together. The engine is another 50 pounds.

With the above load, the boat will continue to accelerate to just over idle speed on the engine and then it will go no faster regardless of the number of RPMs. I would estimate this to be about 5 or 6 knots.

Will changing the prop to one with more or less pitch have any effect, or are we underpowered?


David M 21-09-2008 12:41

Are you underpowered and is your prop the correct size, are two different questions therefore needing two different answers. :)

If your outboard will not come close to reaching its maximum RPM then the engine is also not reaching it maximum horsepower. Your outboard is either over pitched or has too much propeller diameter, or has a combination of both. In this situation, reducing the size of your prop would actually give you more RPM's therefore more horsepower thereby increasing your speed.

What is the maximum allowable horsepower for your boat? If you are at its maximum, then no, you are not underpowered. If you are still not happy with the speed after getting the correct sized prop then I would get a larger outboard so long as you do not exceed the boats maximum allowable horsepower.

To try to answer your question with an educated guess, I think you are underpowered for that boat and over pitched for that outboard pushing that boat with that much weight. I have seen 6, 8 and 10 Hp outboards on other eight foot hard bottom inflatables. Thats just my guess without knowing anything more.

The Mons 21-09-2008 12:50

The maximum hp is 6 hp, the recommended is 4 hp. The prop came with the engine and appears to be the largest that the anti-cavitation plate will allow.

The boat is rated for up to 4 people. I cannot recall the weight limitation.

The prop appears to be cavitating the water. Would that be consistent with being over propped?

What is disturbing is that a friends RIB that is powered by a 2HP Honda with the same load appears to top out at a much faster speed.

David M 21-09-2008 13:30

I would first try getting a smaller propeller and then see what happens. Do you reach maximum RPM then?

Aeration and cavitation are two different thing. Both feel like the same thing. Cavitation occurs when there is too much load on a prop. Cavitation is when the water separates creating a vacuum on the low pressure side of the foil. When the vacuum collapses you feel a vibration....cavitation is not the problem. The oversized prop is underloaded...just the opposite. Aeration occurs when air gets down to the blades. This may be the vibration that you are feeling.

Pblais 21-09-2008 15:03

We have an AB 8 RIB with fiberglass bottom and 6 HP is clearly not over powered but 4 HP is underpowered. I doubt a prop change is going to help you much. It seems the power level is not enough to get you up on plane and so able to go faster. Until you can get up on plane your speed won't change. I think the load rating is something like 750 lbs so if two were kids it would be 4 people. I doubt you are overloaded at all.

David M 21-09-2008 15:10

I would get the 6 horse and be done with it.

highseas 21-09-2008 19:57

If its cavitating its not deep enough in water.I had cavitation problem with My 4hp on 8ft inflatable.Had to cut transom down 1.5 in. .Works great now.

s/v 'Faith' 21-09-2008 20:12

It the RPM's don't go much above idle, your problem is not cavitation. You are certainly over propped, and at such low RPM's not seeing anywhere near the rated hp.

While it is easy for folks who are not paying for the motor to casually suggest that you just 'ditch the motor'.... a prop change might be worth it, and will certainly cost quite a bit less.

Of course this is assuming your outboard is in good condition. Did you just purchase it? is it new? I have seen more then one Yamaha 6/8 four stroke with carb problems that would not allow them to reach rated RPMs... they share a different carb and powerhead but it might be worth getting the motor checked out, especially if it is new and under warranty.

Too bad that your motor is not one that shares a powerhead with a larger hp motor. Many (Nissan/Tohatsu/Merc included) share the same powerhead from 4 - 6 hp. Yours is not one that does, or you could just buy a new carb to boost the hp. (someone may mistakenly suggest that).

Good luck with your problem, I hope you get it figured out.

Ex-Calif 21-09-2008 21:05


Originally Posted by The Mons (Post 207847)
With the above load, the boat will continue to accelerate to just over idle speed on the engine and then it will go no faster regardless of the number of RPMs. I would estimate this to be about 5 or 6 knots.

RPM continues to go up but boat doesn't plane or accelerate.

I suspect its not reaching max RPM.

Need more horses...

What happens solo?

The Mons 22-09-2008 22:00

Thanks for the suggestions so far.

The engine is new. We chose it because of the reasonable weight for a four stroke and the ability to use either the internal tank or an external one.

The original prop is a 7 1/2 x 8. Yamaha also makes a 7 1/2 x 7 that will also fit on the spindle. I suppose I should have a spare prop anyway.

I also think that I will try going by myself to see if the weight reduction that makes any difference. Of course, that won't be a permanent solution.

I will also check the clearance of the prop relative to the transom. I suppose that if this is the problem, I would rather try to get a longer lower running gear unit than an entirely new engine. I not really sure how I could cut down the aluminum transom due to the way it is designed.

Has anyone ever seen an adjustable motor mount on a dinghy?

Pblais 23-09-2008 04:50


I also think that I will try going by myself to see if the weight reduction that makes any difference. Of course, that won't be a permanent solution.
That would a good test. I'm sure you don't want that as a permanent solution though. Sorry honey it's to much weight. So long!

mobetah 23-09-2008 05:37

Why not borrow your buddy(s) down the dock's 6 and/or 8 hp, for a few minutes, just to see what difference there is?

amytom 23-09-2008 06:33

I also have the Yamaha 4hp. On my Achilles 9'6" inflatable I can only get on a plane if I'm alone in the boat. Even then, it will cavitate and I drop off plane right away. Is this because I don't have a rigid hull?

Ex-Calif 23-09-2008 07:42


Originally Posted by amytom (Post 208405)
Is this because I don't have a rigid hull?

I'm not even gonna touch this one :devil:

OK - Fundamentally, if you want it to go fast, ya gotta get the dinghy on plane. Getting it on plane takes horsepower.

Otherwise you have a displacement hull dinghy and it goes at hull speed.

I don't know how the manufacturers rate dingys and motors but I am guessing the low to middle end (hp) creates a putt-putt dinghy. The high end of the horsepower curve creates a planing dinghy - just a guess as I contemplate how marketing guys think...

sandy daugherty 23-09-2008 08:36

The Mons: You are underpowered AND overpropped. A prop change won't get it. For your load you need an eight horse fourstroke, propped to peak at about 15 mph. There is a formula, but it needs to know your slippage, a function of boat, engine and prop. You just might be able to get on plane by yourself, or someone even lighter might be able to, but thats not what you got the dinghy for.
Fortunately, Yamaha makes an eight horse engine. Lets hope the dealer will let you trade up. If you get the high thrust version with a 2.92:1 gear ratio, you will have several prop choices, and it will pull (small) stumps. Take a nerve pill before you ask the price.
Amytom: When a soft bottom dinghy approaches planing speed, a lot of air finds its way under the boat, in uneven lumps. You will feel it inside as the bottom feels like its running over ruts. If the engion is mounted a little high on the transom, or raked too much one way or the other, the prop will cavitate, the engine will overspeed, and the little electronic brain will pull the plug. Sometimes the prop will get trapped in a recirculating donut of air or froth, and overspeed while developing no thrust. here are some solutions in order of expense:
Increase the tube pressure to stiffen the dinghy and reduce the corrogation effect. Set the 'angle of dangle' on the engine to perpendicular to the water when level. Lower the engine on the transom as far as it will go. Test in calm water. Test in 6" waves. Decide if you want to go fast bad enough to buy a hard bottom (check out the high pressure floor models if you need to store the dinghy aboard, and a bigger outboard. I use a 6hp Tohatsu on my plywood-floored West Marine 310, and it will plane at 12 mph with just me aboard, but (quothe the raven) never more.

All times are GMT -7. The time now is 03:00.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.