What effect does motor height have on attitude (bow lift, porpoising, etc)?

JimInAlabama

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G3 Eagle 175 (920 lbs), Yamaha 90 four stroke, Yamaha K series aluminum prop - 12 5/8 x 21

WOT 5,900 rpm - 43 mph, good hole shot, comfortable cruise 3,800-4,000 rpm @ 26 mph or so. I like everything about it except excessive bow lift at slower speed, it comes off plane around 3,200 rpm and bow lifts so much I can't see over it.

Motor is in second hole from top, cav plate is dead even with keel. What effect does motor height have on attitude? Would raising the motor have any effect on slow speed bow lift?

Now the rest of the story! I installed a SE Sport 400 no-drill hydrofoil thinking that might help, and it did help with bow lift, it stays on plane down to appx 2,800 rpm. But it had a terrible effect on performance, I can't trim up enough to get it out of the "plowing" attitude without porpoising like crazy.

Trim tabs are not an option for me.
 

JimInAlabama

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Thanks, but there’s nothing to move. Gas tank and both batteries in the stern. All gear is in forward boxes.
 

Sea Rider

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What's the max wot rpm range of that motor is it over 6 K ? Was the max wot revs achieved while solo boating or with how many up ? With motor running trimmed to 90 deg on plane, at which lower leg height is water flow skimming by, is it under small upper plate, over AV lower plate or right in middle between both 2, for that will need to visually check side of motor at plane. Test on flat calm water cond.

Happy Boating
 

jimmbo

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As the engine is mounted higher on the Transom, the ability to raise/lower the Bow by trimming drops. Less leverage. As the boat gets on plane, the bow will be at its Highest, dropping as speed increases. Your options are, move weight forward, Operate with engine trimmed in all the way, Lighter engine(this is where 2 strokes had a real advantage over the 2 ton 4 strokes). You might be able to find a prop design that will lift the Stern at any given trim setting. These props were originally intended for use on Racing Tunnel Hulls to reduce them from flipping over backwards. They will kill top speed though
 

JimInAlabama

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Have the engine raised to the last or next to last hole from bottom. Went through this with the boat I bought a couple of years ago, and again with the new boat my bother bought a few months ago. here's what I did.
https://www.thehulltruth.com/boating-forum/820886-prop-selection-advice.html

Thanks Wes. Did you notice any before/after difference in how slow you could go before you came off plane? (I'm ok with overall performance, would just like to be able to go slower and see over the bow, without killing top end performance!)
 

WesNewell

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Didn't pay close attention to that. However, not having to trim the engine up near as much causes a more forward thrust which keeps to bow down lower during acceleration. My engine was originally in the middle hole. I mounted mine in the last bottom hole, raising it 1.5". My brothers was mounted like yours and he taised ti to the next to last hole, still 1.5" and he's happy with it there. His top speed increased ~10% and ran smoother without having to trim up as much.
 

roscoe

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I have similar problem with coming off plane.
Raised the motor, plate is now 2" above bottom of hull.
The drop off plane speed fell a bit. Not much, maybe 200 rpm difference.
I believe the plane/no plane issue is more a hull design issue.

I want to try smart tabs which I'm sure would help, but kicker motor, transducers, and boarding ladder are all in the way.
 

ahicks

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As we know, everything boats is a compromise. As such, you might consider a 4 blade prop. It's likely going to cut into your top end, but it should help the boat plane out quicker, and carry it on plane at lower speeds.....

With the extra lift, I would think the bow would drop sooner, at a lower speed.

And that whale tail has to go. The cavitation plate is just not designed for that kind of stress. Those things cause them to crack/break off all the time.....
 

Faztbullet

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The small diameter K style prop is hurting you..You need prop that holds water. Need something like a Talon in 13.25-13.50 X19....
 

Texasmark

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As the engine is mounted higher on the Transom, the ability to raise/lower the Bow by trimming drops. Less leverage. As the boat gets on plane, the bow will be at its Highest, dropping as speed increases. Your options are, move weight forward, Operate with engine trimmed in all the way, Lighter engine(this is where 2 strokes had a real advantage over the 2 ton 4 strokes). You might be able to find a prop design that will lift the Stern at any given trim setting. These props were originally intended for use on Racing Tunnel Hulls to reduce them from flipping over backwards. They will kill top speed though

Right on. Wink!

I am currently "diddling" my engine height on a newly purchased 2002 boat so the cause and effect are fresh on my mind. I bought it with the engine in the 4th hole down; clamp bracket about 2 inches above transom seating surface. Don't know why PO had it up there as that and the prop installed (not sure it was what the PO ran) was way over pitched.

Currently I have it in the second hole and ¾" off the transom with the prop pitch reduced from 22 to 17 and I think I finally got the RPMs where they belong and expected boat performance. Running at mid trim and WOT new water is just over the AV plate and control is what it should be. A few bumps UP of the trim and the bow lifts nicely out of the water. A few bumps DOWN and the bow contacts oncoming chop and splits it nicely for a soft (slower) ride.

Once the anti-vent plate ceases to dip down into new water (water flowing out from the transom) you will essentially lose the ability to lift your bow (front of the boat). Trim positions will have minimal affect as compared to having the plate in the fresh water. Any bow lift occurring at-near WOT when set high will depend solely on hull design and thrust angle.

With the AV plate even, you are currently able to dip into that water with the front of the AV plate and have a pronounced effect on it's attitude. With that said, your best bow down configuration is max neg trim....midsection up against the boat transom.......tilt pin removed. Also firewalling the throttle to get past the bad spot as fast as the boat will let you. As you come up on plane and the bow falls, trim out as you go till you get the desired boat "attitude" relative to the speed and water conditions.

Whales tales, trim tabs that mount to the transom, and 4 bladed props made for "stern lift" (stated in the prop specs) will aid you from where you currently are.
 

Faztbullet

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Tex... The AV plate has very lil to do with lift and only contributes mainly at planing...You know that the actual LU housing/ prop an hull are the lifters at speed???
 

JimInAlabama

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I have similar problem with coming off plane.
Raised the motor, plate is now 2" above bottom of hull.
The drop off plane speed fell a bit. Not much, maybe 200 rpm difference.
I believe the plane/no plane issue is more a hull design issue.

Thanks Roscoe. I'm sure you're right, most likely it is what it is! I am curious though, did you see any performance improvement after raising your motor?
 

Texasmark

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Tex... The AV plate has very lil to do with lift and only contributes mainly at planing...You know that the actual LU housing/ prop an hull are the lifters at speed???

Yes sir I do. The point here is tighter control over how much hull is wet (drag) as a function of trim angle.
 

JimInAlabama

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In case anyone is following this thread, or finds it in a search ........

I took WesNewell's advice and raised the motor as high as it would go, on the '05 Yamaha F90 that's the fourth set of holes and that helped a lot, i don't think the hydrofoil was getting up out of the water. The trim's still a little touchy at top speed but it's manageable. WOT rpm's 6,000 and top speed 44 mph.
 

Texasmark

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In retrospect, whales tales are for cruising/hole shot, not WOT performance. For that, they are just something else to add to the drag factor. On trim being touchy, bet you look at how much of the tail gets wet at your different WOT trim angles and you'll see the reason. On 6k rpm, I ran a 115 hp 1988 Merc I6 Tower of Power (rated at 5500 max) at 6k every chance I got for 7 years. Sold it to my SIL and last time I checked, many years ago now, he was still running it. It loved rpms.
 
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