Determining Correct Engine Height

[RayB]

Is there a formula I can use to determine the correct engine height for my boat. I have been speaking to other Trailcraft owners and it appears that my engine may be a bit low but I thought that if someone had a general rule used to determine engine height then it would help make up my mind.

I guess I could use trial and error but was hoping to be more scientific about it.

Also, what are the signs that you have the engine too high?

My boat - Trailcraft Profish 465
Engine - Mercury 60HP 4st

cheers
Ray

[blaze]

Water rises about 1" for every 12" you go rear of the transom. This varies with the displacement weight of the boat ie: more for glass, less for a tinnie. Rule of thumb for motor height is level to 1" above the planing surface of the boat. Too high will get cavitation on turns and a tendusy to chine walk, low low will create lots of drag and excess spray.
cheers
blaze

[RayB]

Thanks blaze! Good info!

I have researched the Boating forum and I have found a couple of statements regarding correct adjustment for motor height. do you guys agree with these statements? I realise not every rule will suit every boat but are they good rules of thumb?

- cav plate on motor should sit about 1 inch above the lowest point on the transom.
- If you have a foil attached then you might have to raise the engine higher again

Let me know what you guys think. I will be raising my engine tomorrow and would appreciate any comments you might have

cheers
Ray

[blaze]

I hate foils
I have never had one on a boat, so I have no experence with them. Hearing the horror stories does it for me. My thoughts are that the individual outboard manufactures spend millions of $ on R&D and have not made larger cav plates, if any thing they are smaller. There has been a lot done to reduce drag of legs, not increase it. they have to put extra pressure on cav plates (apply principle of leverage). They are used to mask other problems ie: power to weight, poorly designed hulls.
just my ramblings and plenty will disagree
cheers
blaze

[RayB]

Thanks Blaze and I am still undecided about the foils myself!

I am hoping we can avoid turning this thread into an 'I hate Foils' thread and I am still interested any more thoughts on the motor height adjustment

cheers
Raymond

[Wooly]

Ray

Just been thru this myself, had my O/B cav plate 20mm above keel (18ft viking with 150 O/B), handled like a dog (with or without fin). Droped the motor 20 mm to level (one hole) and now is a totaly different boat, jumps out of the hole, better trim range, faster, better steering but best of all no more porpoise. All I can suggest is worth a bit of trial and error. Just remember when you do your sea trials that how load your tub up as you would normally as weight can make a big difference.

As I have said before I didn't think 20mm could make such a difference!

And re the fin... I'm with blaze...

Good Luck

[Roo]

- cav plate on motor should sit about 1 inch above the lowest point on the transom.


Am I reading this right??
I thought the cav plate should be between level and 1" BELOW the keel.?????

I'd have thought being above the keel height would cause the prop to ventilate.??

Cheers, Roo.

[RayB]

thanks for the replies. I took the boat to a dealer this morning (different from the one that sold the boat) and they suggested that I should raise the motor at least 2 notches which is about 40 mm. I will raise the engine and give it a run on Saturday. If it is not right then I can always take it back down 1 notch.

cheers
Ray

[Forrester]

Ray
My Honda book says my cav plate should be 0-2 inches below the bottom of the hull and it's close to 4 inches so looking to do the same as you. Would be interested in the outcome. Please publish your findings for all of us.
Roy :)

[blaze]

they make props for racing boats that are only half submerged, image the rooster tail off that. they get an increase in speed because of less drag (gotta have the right prop.
Each boat will be different and rrial and error is the best way to find the correct postion. Remember to try different trim positions too.
cheers
blaze

[RayB]

I look forward to posting my results on the weekend.

Blaze, my next task is to replace my prop as well. From what I have heard the right brand / type prop can also increase performance. I will get the motor height correct first and then purchase a prop, probably solas unless anyone can recommend a better supplier.

[weipatopender]

I am looking at getting a new ss prop for my 455 topender yam 60f rang solas thay said the first thing I should do to determine if the motor is the right hight was to load it like I would do for a tippical day get it out on calm water go flat out trim to get the boat to its sweet spot and at that point the cav plat should be just under water 1cm I was told that if it is an inch down I would be loosing about 250rpm my moter should wind out to 5800 6000

[nevd]

I am looking at getting a new ss prop for my 455 topender yam 60f rang solas thay said the first thing I should do to determine if the motor is the right hight was to load it like I would do for a tippical day get it out on calm water go flat out trim to get the boat to its sweet spot and at that point the cav plat should be just under water 1cm I was told that if it is an inch down I would be loosing about 250rpm my moter should wind out to 5800 6000
I agree on the engine revs quote, but I don't agree that you need to have the AV plate under water at WOT. Any good stainless prop on a boat without keel extrustions close to the transom will allow the AV plate to be run 20 or 30 mm above water at WOT. You will need to trim in a little in rough water and when turning sharply, but some prop ventilation should not be avoided by having motor mounted too low.

[RayB]

The Results!!!!!!!!

I raised the engine about 40mm and I have seen some results. I ran 4 diferent tests. all tests were carried out with 2 large blokes on board.
Test 1. Foil installed but no bimini
Test 2. Foil installed and bimini installed
Test 3. no Foil and bimini installed
Test 4. no Foil and no bimini

Test 4 obviously showed the best results I was able to increase rpm at WOT by approx. 200rpm and also increased by top speed by approx. 2-3knts. The results are compared with the results from last weekend when the motor height was lower.

Other interesting results.
- The foil SE Sports 300 had very little impact upon top end speed with the difference between Test 3 & 4 being only 1knot at WOT.
- I believe raising the engine has given me shot out of the hole but it is marginal.
- the foil still makes a big difference in getting onto the plane quicker although I will probably only use the foil if I have a particularly heavy load in the boat.
- At various rev ranges the results were very similar in all 4 tests with the biggest difference being speed at WOT.
- with 1 person on board I was able to mx out at 6000rpm and with 2 blokes on board I got to 5800rpm which is about right for this engine.

picture of current boat height

[Wooly]

Ray

Good to see you got a result. Nice photo...by the look of ot you now have the cavplate level with the bobttom of the hull?? is that right??

I have always hated biminis, always in the way, now you have got a good reason to get rid of it or a least store it away until summer!

cheers

[Stumpy-USA2]

Over here we always set our engine to begin with where the centerline of the prop shaft is 3.5" below the bottom center of the hull and adjust up until your water pressure drops. Next you mark your spot on the mounting bracket and drop the motor in 1/4" increments until the water pressur comes back up then you are set. I have mine set a 2.75" below the hull, for every 1/2" lower it looses around 200 rpm. The difference from the factory setting and where it is now in night and day. I gained around 6mph at WOT and cut the time to plane down by close to two seconds. It also eliminated most of the chine walk at high speed and most of the bouncing at low speed.

Stumpy