Outboard height with setback

[chevy355mark]

Yes I have used the search function. The general rule is 1” per 12” set back. Well my outboards are 1.75” up from the bottom at a 24” setback. It’s a 23ft boat, 24 degree bottom with a small pad and twin 250 merc efis in 30” shaft length. The boat porpoises like crazy and it’s hard to handle. Will moving them up another 2” for a total of roughly 4” up help or hurt?

I’m about to fabricate some simple vertical jack plates from 1/2” angle aluminum and I’m debating if it’s worth my time and effort. Yes the bracket and transom can take the additional load, it’s all been way overbuilt over the years of previous repairs.

[Jimboat]

... my outboards are 1.75” up from the bottom at a 24” setback. ... Will moving them up another 2” for a total of roughly 4” up help or hurt?

what part of your motors are 1.75" above bottom of pad? of local vee surface? Where are the CL of torpedo on lower units located, relative to local vee surface?

[chevy355mark]

The cav plate is exactly 1.75” from that point of the hull bottom. So to clarify, where the centerline of the outboard hits the v bottom, that point is exactly 1.75” lower then the outboards cavitation plate.

the outboards are at the v portion of the hull which is just a strait 24 degrees, no variable deadrise or anything funky. The pad may be 5” wide and 14” long, it’s not much

[chevy355mark]

Few pics always helps

[Jimboat]

The cav plate is exactly 1.75” from that point of the hull bottom. So to clarify, where the centerline of the outboard hits the v bottom, that point is exactly 1.75” lower then the outboards cavitation plate.

Difficult to see your engine height from the picture angle, but it appears that your engines are mounted quite low. Raising will probably help susceptibility to porpoising.

Note: Lower Unit height is best measured with reference to the CL of the 'bullet' or 'torpedo', not the cavitation plate, since thrust comes from propeller shaft, not the cavitation plate. Height is the measure to local planing bottom surface.

Also: see article on 'Porpoising' which may be of help.

[chevy355mark]

So where should my nose cone cl fall then? Even with the bottom of the V?

if I’d benefit from a full 5” up I’ll have the shafts cut and go conversion. Anything less and I’ll do the adapter route as I don’t want the motors any lower to the water

[Jimboat]

try higher using small increments. watch for sufficient water/cooling pressure. So where is your nose cone CL now?

Also check out articles on 'Porpoising' and 'Boat Static Balance' and 'Dynamic Stability'

[chevy355mark]

the nosecones center is is 8" away from the cav plate making them 6.25" below the local vee surface.

[Merc 2.5]

A good rule of thumb is to get prop shaft 3 1/2in below pad( running suface) then watch pressure , its seems to work great for starting point with no low water pick up, even with pad will be to high and low.water pressure

[FMP]

Can you remove 5" extensions from those legs and machine the drive shafts then drop down a hole or two ? Every time I see those monsters I lose count of cav plates.
Is that an old stepside?

[chevy355mark]

These have low water pickups anyway. I’ll get out there and really do some more accurate measurements. I really don’t want them any lower. If they were 2.5s I’d say no way do I want them up in the air but these are pretty heavy and the boat squats really deep coming off plane especially in rough seas or running a heavy following sea.

FMP, that’s an 86 narrow frame dually with a step side bed. These 1998 outboards are sadly the newest piece of equipment I own... by quite a bit lol

[Jimboat]

the nosecones center is is 8" away from the cav plate making them 6.25" below the local vee surface.

You probably have lot's of room to move engine higher. Raising engine will likely help your porpoising susceptibility. Be sure to raise in small increments, checking/monitoring water/cooling pressure all the way.

[big tom]

Did you buy the boat set up like this or did you just put the motors on?. Take a 2x4 and lay it up against the hull and side it out til it hits the drive and see where the top of the 2x4 hits the drive with the drives all the way down. Seems if they where to low they would be driving the bow down. JMO.. Could also need a bigger dia prop for your application.

I have my motors high. It only takes a couple bumps of the trim switch to trim, than one more and she starts to porpoises.

[FMP]

If powerhead height off plane and trolling in good swells then yeah otherwise the shorter leg etc but it looks from here if the 5" spacer was out of the leg lowers could be a lot closer to good height +/- a hole.
Need two 25" shafts and water tubes etc.
Going to look high up.
86 !

[chevy355mark]

I’m starting to lean towards doing the 25” conversion the more I look at it and thing about it. I’ll post the final product once I pull the trigger