Outboard motor mounting

How can you tell if the motor has been mounted to low on the transum ? What would the effect be if it is ? I have been looking at a friends boat and his motor is mounted 2 inches higher and is the same kind of boat only mine is 1 year newer. He has a 75 and mine is a 90 . Does the weight of the boat have anything to do with it ? Mine is a tournament pro and his is not. I can only trim mine very little or it will start to proportion. Could the cavation plate be digging because it is to low ? Any help would be greatly appreciated. Boats are Alumacraft 2000 and 2001. If you have a setup like this would you please let me know where your motor is set at ?

Great question but sorry I don’t have a good answer. Just one comment and question. Do you have a four stroke engine on your boat? It seems to me that some boats (not sure about the Alumacraft)cannot keep the bow from porpoising when you trim up with a heavy four stroke engine. I believe part of the problem is some boats are designed with the fuel tank in the stern and the heavy four stroke engine puts too much weight in the back of the boat. I noticed a big difference (improvement) on my new 16 foot Smokercraft that has an under floor mounted fuel tank. I no longer have bow bouncing problems when I trim. As far as mounting the engine high or low on the transom, this is an excellent question and I look forward to an educated answer from someone. When I rigged boats 30 years ago as a kid, taking directions from adults, we set them all down on the transom and bolted them on. That doesn’t appear to be the way it is done today. – jim

As for your question yes they are both 4 stroke motors. I beleave both have a in floor gas tank . Mine is the tournament pro and his is the model just down from there. Only differance is mine is a Suzuki and his is a Merc. Mine MUCH MUCH MUCH quieter.

My dealer installed my motor on the third (middle) hole. My boat is a 1750 C-liner Fishhawk and I run a 90HP Suzuki 4 stroke. Some dealers mount 1 hole lower on the same boat. Not sure what the performance differences would be. About all I can do to answer your question is to tell you how my boat trims in the third hole mounting position.

On the Fishhawk, the feul tank is below the rear casting deck and the Suzuki motor weighs just over 400 lbs (lots of weight towards the rear).

I always get up out of the hole with the motor trimmed all the way down. If the motor is more than 1/4 up (according to my trim gauge), it will cavitate on an agressive start. I usually start to trim up immediatly out of the hole. At lower speeds, I usually run trimmed 1/4 to 1/2 way up. Any further and she will start to porpoise. At full throttle, I trim to about 3/4 up. If over trimmed, the boat will not porpoise at full throttle, instead it will rooster tail and start to loose top end speed. For whatever reason, at full throttle, the amount of trim on the motor does not have a big influence on how my boat wants to plane. The biggest difference that you can see is a few more or less mph on the speedo and a slight impact on steering torque.

In comparison to the boat that I owned before, I would say that this boat is much less prone to porpoise but more prone to cavitate. I had a 16′ Smokercraft Pro-mag with a 60hp Mariner 2-stroke before. The motor was mounted on the top hole which in hind sight was probaly sitting too low in the water. I could really get that boat to jump out of the water at any speed if I was over trimmed. On the other hand, this motor would rarely cavitate and was a real dog out of the hole to the point that I decided to install a hydrofoil.

Hope this helps…

Normally the less of the lower unit in the water the better the performance. There is less drag, so hole shot and top end speed usually improve. Generally, the cavitation plate should be even with the bottom of the transom. The more of the lower unit in the water the greater the drag and typically slower performance. However, your boat may run best that way. If you raise the engine one or two holes you may improve hole shot and top end speed. But be carefull, if it is mounted too high, at higher speeds, you may not get enough water pick up to cool the engine.

I asked my dealer to raise the engine one hole and the performance was noticably better. Faster and quicker.

If you are having porpoising problems try moving weight around in your boat. One way is to fill the front livewell and see if that reduces the porpoising.

Also, four strokes typically have slower hole shot and top end when compared to two strokes.

All hulls are a little different and exhibit unique planing and high speed running characteristics. Some hulls will porpoise when trimmed out too far, some will start to get squirrelly and skate. stainless props typically will allow you to trim out further at high speed before the boat gets loose. SeaArks like mine have a “wedge” at the back of the hull to force the nose down at speeds above @ 27 mph witha typical stern heavy layout. I set up my interior custom, with a bow fuel cell, and the console up front…so I ended up having the dealer grind the wedge down, so I could get the front of the boat out of the water for more speed. Bottom line every situation is alittle different and usually takes somework to get as close to perfect as possible. As a rule, I agree that the cavitation plate should be even with the hull bottom, for starters.

Jason.