Transom height performance difference

[oneoldude]

My boat has a 19" transom rather than the typical 20" transom. As a result the cavitation plate of my motor is 1" below the bottom of the boat's bottom. How does this affect performance? <br /><br />If I were to mount the motor so that the cavitation plate was exactly at the bottom of the boat what would improve or get worse re: performance?<br /><br />My boat's transom can be seen at the URL below.

 

[jim dozier]

Re: Transom height performance difference

Generally the cavitation plate should be no lower that the bottom of the hull. You can usually raise it some above that depending on the individual hull. It can have a dramatic effect on top speed particularly at higher speeds. If your top speed is 25mph it probably won't change much but if your top speed is 40mph or more, getting the engine height right is critical. Play around with it. Higher is better so long as the water pickup is working (water pressure remains OK) and the prop isn't ventilating.

 

[walleyehed]

Re: Transom height performance difference

Many benefits can be found be testing the engine heigths. I would recommend you raise the engine one hole at a time and test. It's very simple to loosen the bottom bolts and remove the top 2 to do this.<br />By lowering the drag of the gearcase you can acheive more RPM and in most cases, a little speed too.<br />The handling charactoristics should improve as well.

 

[steelespike]

Re: Transom height performance difference

I saw an old test on a Penn Yan Swift They raised the motor an inch and gained almost 3 mph.<br /> Forget what the final figure was but was around 40 mph.

 

[JasonJ]

Re: Transom height performance difference

I have gained dramatic changes in rpm, top speed, and economy with massaging of the motor height. I plan another 1/2 inch adjustment, which will then allow me to step to a 19 inch prop. The adjustments I have made so far have me at 6000 rpm with just me in the boat, wide open. I figure if I can get a few hundred more rpm, I can then step to the 19, which will bring me back down around 6000 rpm, and should give me a speed increase. I have spent a lot of time tweeking my setup, as I don't have power trim. I am getting closer to what I think should be optimum. I have also had to do a lot of set up change because of adding more stuff which equals more weight and different load distribution.<br /><br />If you can get your cav plate even or a half inch above the keel, you will then be able to tweek with trim angle and prop. You will need an accurate tach though, otherwise you will have no idea if you are making progress with your tweeking. Good luck....