Raising Engine

[Tuna Man]

I finally took a ride on our 2002 209 today. I am almost certain the engine height could be raised at least one hole after running it for a few hours today. On the same subject, I've been considering raising the engines on our 300 for a few years now.

I'm hoping you guys could give me a little insight with regard to real world advantages of raising an outboard (or twins). I gather I'll gain a little speed (how much), increase fuel economy (how much), and increase WOT rpm (how much). I assume that if I end up raising too much the prop will easily blow out.

Please share your ideas, comments, etc.

Thanks

 

[seasick]

I finally took a ride on our 2002 209 today. I am almost certain the engine height could be raised at least one hole after running it for a few hours today. On the same subject, I've been considering raising the engines on our 300 for a few years now.

I'm hoping you guys could give me a little insight with regard to real world advantages of raising an outboard (or twins). I gather I'll gain a little speed (how much), increase fuel economy (how much), and increase WOT rpm (how much). I assume that if I end up raising too much the prop will easily blow out.

Please share your ideas, comments, etc.

Thanks

There is no magical rule. If you motor is at the best height now, raising it can lead to poorer performance. Before anything, take a look at where the cavitation plate is relative to the water line at cruising speed and at proper trim. It should be a tad below the water, maybe an inch or so. If it is too high you will reduce thrust, create cavitation and add slip. Too high is worse than too low. The optimum height is dependent on a lot of factors and everything is a compromise.

 

[Tuna Man]

I've read that ideally you should be able to "blow out" prop when trimmed way too high, I cannot blow out prop(s) on either boat if I tried. On the 209 the cavitation plate appears to be about two inches below the water at cruise and WOT. I think I'll go up one hole and test it out. I'll post results when I get it raised and test it out in the next week or so.

Yesterday the 209 hit 42mph at 5300rpm with fresh bottom paint, five adults, full gas tank, full live-well and about fifty pounds of gear.

Any other comments or suggestions?

 

[onoahimahi]

I've always heard that the anti-ventilation plate should be above the water at cruise, not below it. Here is mine at 3 RPMS (4500, 5000, and 4000). I've heard that the RPM can change by 200 or so when the engines are raised to pull the A/V plate out of the water.

https://www.youtube.com/watch?v=9XBsCt_9bgc

 

[onoahimahi]

There is a better video here - three different heights for an F100:

http://www.etecownersgroup.com/post/Eng ... eo-6377951

 

[REBThunderroad]

Although not a GW my other boat is an 18 foot Boston Whaler Dauntless with an F115. I had a WOT of 38 mph at 5800 RPMs and hard to steer. The cavitation plate was approximately 1" below the bottom of the keel. IMHO Singles are easier to figure out then twin motors. I raised the motor where it is now about .5 inches above the keel. Best way to measure is to take a ruler and hold it along side of our cavitation plate (trim motor so cavitation plate is level with the keel) and see where is lines up on the keel. Ideally you want to be even or within an inch above the keel. I was told that you want the motor as high as possible without blow out. The result was I picked up 200 RPMs (5800 to 6000) and 2 mph on top end. The boat comes out of the hole much better and steers much quicker. Most of this is due to less drag and findning the sweet spot.

On a side note I have a 2005 Marlin 300 with F250's and currently running 15.5 x 19 Yamaha SWS II 3 blades. I am on the second hole (I can go two higher) which is perfect for the 19' pitch 3 blades. I am going to buy 15 x 18 " Powertech 4 blades once I buy the props I am going to raise the motors. I was advised to raise them all the way up and see how they do (as 4 blades perform better higher). If two holes is too high then I know to lower one and seal back to the boat. I think the rules is raising about an inch can increase RPMs by about 200 RPMs although each application will have variables.


All this knowledge (LOL) came from researching on the web, Prop Gods, and the tech at powertech props. I look forward to getting the new 4 blade props.

 

[Tuna Man]

Ono - my 209 outboard it definitely way too low. If I use the second video as a reference, my motor is about three or four inches too low. I thought the first of the three segments of the video was acceptable. Now I'm thinking I should raise two holes for starters.

REB- makes sense, you've been studying more than I have that's for sure. Ken at Propgods suggested I raise the motors on the Marlin when I switched to the four blades a few years ago.

Thanks for the input, sounds like I may gain about 200 rpm at WOT and maybe one or two mph. IF I get all three outboards done, I may save enough gasoline this season to buy a new Grady key fob, or perhaps a Dunkin Donuts cup of coffee. I'll post before and after reports when I make the changes.

 

[seasick]

Remember that the full tank of gas does lower the hull and therefore the depth of the cav plate. Just going back to the stern to loo does the same thing. What I am trying to say is that that boat is relatively light and the weight shift and load has a more dramatic effect on the position of the motor tailshaft.

It is OK to experiment with motor height but be careful since as you lose weight the motor will rise as the hull does and you could end up with blowout. In addition, if the motor is too high, some of the thrust of the prop will be directed at the lower transom instead of under it. That will make the setup less effective.
Of course the hull itself is the biggest factor for drag. The more hull touching water, the larger the drag. That is why the trim is so important. The cavitation plate is a small component of total drag
You may want to calculate your slip. That will tell you to a point how efficient the entire setup is.