Classic SeaCraft Community - Prop for 23 SeaCraft with transom-mounted Merc 250 EFI

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Quote:

Originally Posted by Bigshrimpin (Post 271147)

EZ . . . The simple answer is setup and weight :) My old 225 mariner carbed engine was a 1997 and had the 1.75 ratio lower. Both 3.0L 225hp and 250 are closer in hp rating than you might think. The later mercury 225EFI's were known to dyno at 242hp. You can run those Rev4 props higher without blowing out . . . less lower unit in the water means less drag (and more rpms). See the Hijacker fixed jack plate on the back of my boat with the 225.

100lbs in the wrong place can slow you down a few mph. Every 100lbs extra will make your top end suffer. Waterlogged foam in stringers can add several hundred pounds.

T Tops can be the absolute worst performance killers!!! Poorly designed Tops will catch the wind and act like a giant parachute. Just picture a 4x8 sheet of plywood flying through the air at 40mph. Tilt the front of that sheet up 5 - 10 degrees into a 15mph head wind . . . what happens?

We are talking about 25year old motors. If we were comparing both engines using the exact same hull in a controlled environment then we could say more definitively that your 250efi was tired. 46mph is respectable with a 250.

Here's a video of my boat with a 1987 vertical reed chrome bore 2.4L 175. Running light the boat would tickle 43/44mph. Those 2.4L engines weigh under 400lbs. Early fingerported 2.4L 200hp horizontal reed mercs would dyno at 218hp from the factory. That vertical reed 2.4L was real strong too . . . I'd bet money that engine was over 200hp on a dyno (despite the sticker on the cowling) and had good bottom end torque. (here's a few mods for the 2.4L . . . https://www.chattanoogafishingforum....=39864&start=1)

https://www.youtube.com/watch?v=jg7cdv1w1vE

I don't think weight is an issue (the boat was fully rebuilt), but it does have a large t-top. That and the fact that the engine could still go up 2 holes probably.

BTW, going by your picture, looks like you have a 25" shaft motor on a 30" transom (at least mine was exactly 30" after I filled the "second notch" in). That could explain how you were getting those rpms!

Looks like I'm gonna raise the motor and try the REV4 in 17 pitch first.

There's no worry that the 18 pitch Eco and 19 pitch Highfive won't decrease rpms?

Large Prop Diameter is NOT your friend if you want speed and to turn max RPM's. So if you decide to go with an enertia, you want the original smaller diameter enertia NOT the ECO enertia ;) In my experience the smaller original enertia turns 200 - 300rpms more for the same pitch mirage plus. It has less stern lift, but it's a faster prop. If you turn a 17p mirage plus you can turn a 19 highfive. Highfives are small diameter 13.5" vs mirage plus 17p which is 15.5". Highfives also have thin blades like the enertia (original NOT eco).

See this thread: It's a sticky in the performance section.

http://www.classicseacraft.com/commu...ad.php?t=28564

and read this one twice

https://www.boatingmag.com/boats/choosing-right-prop/

I run a Rev4 18” pitch on my new Merc 300-V8 and love it! The 4 blade Rev4 is a stern lifting prop and has great bite on take off and doesn’t cavitate or blow out even in hard turns. I can turn 6000 rpms and max out at 53 mph light on my 23’ Sceptre. I tested 2 other props they didn’t perform as well.

After you get your engine height correct, you might try a 17” pitch Rev4 to compensate for lesser horsepower (250 vs. 300)

Quote:

Originally Posted by rdoaner123 (Post 271149)

BTW, going by your picture, looks like you have a 25" shaft motor on a 30" transom (at least mine was exactly 30" after I filled the "second notch" in). That could explain how you were getting those rpms!

I had a 25" engine on 25" transom. As mentioned above I ran a fixed jack plate (see picture) that raised the engine to 27" and put the cavitation plate about 2" up. It was dialed in pretty well. I could NOT run a mirage plus at that height.

I run my Rev4 on a 27” transom (built 2” taller than a standard 25”). 25” motor.

Quote:

Originally Posted by Bigshrimpin (Post 271158)

Thanks Bigshrimpin (as well as all the others) for all of the advise, you have been incredibly helpful! Hopefully in a couple weeks I'll have it all dialed in and will report back to you guys.

i have a 17p mirage plus getting dusty once i put on a 17p rev4 never went back. probably could have gone higher in height. the 17P blew out on hard turns. i lost 2-3mph and 200-300 rpm i think but don't have hard numbers at hand. wot speed less worried about than making sure engine can get into its proper wot rpm's. i can spin to 6k/6100 rpm heavy loaded trimmed up and more light load. 6400 is max. verado 275, 650lb motor. t-top.

looks like next winter I need to think about a pilot house for another major project. i've never ever seen over 42/43 light load.

have a new (to me its a 2020 merc 300) engine this season so prob go to a 18p rev4. that highfive though is interesting....hmmmm.....i like the stern lift of the rev 4.

Bigshrimpin How is the high 5 prop running in seas east of the cape / offshore compared to rev4? planes out about same speed?

that is one sweet pilot house Bigshrimpin! wowza! is that a seamark? they are out of business now? :( went looking around for them a few years back came up blank.

I should mention, I've been noticing the prop sometimes partially breaks free even when I'm going in a straight line and increase rpms. The prop shaft seems straight, so I'm not sure why this would be

Attached is a picture of the lower unit with slightly positive trim at 3600rpm. I circled the back of the anti-ventilation plate in red. You can barely see it through the water. Should I go up one hole or two?

Quote:

Originally Posted by Bigshrimpin (Post 271147)

You make an excellent point that few discuss.

We have engine trim and trim tabs.
We have jack plates.
We have prop design.

Nobody talks about T tops.
Why not?

I have not seen a hardtop designed like a chord or wing
if you will. Think about a cutaway of an aircraft wing.
Flat on the bottom and curved on top.

Wings do not create lift.
Air wants to remain constant.

Since the distance traveled on the top of the wing
is greater than the bottom, the air has to speed up
to remain constant in its mass.

When you have an increase in air speed
you create an area of low pressure.

The wing moves from the area of high pressure
to the area of low pressure.

This is called lift, which is just an easy way of explaining
Bernoullis principle of hydrodynamics.

A t top design where you could optimize the angle of attack
and lift with trim of said top.

This is usually where Denny chimes in and says
he knows just enough to be dangerous.

Now, certainly this opens up another avenue of design failure
and operator error, but it is intriguing.

Cheers,
GFS

Quote:

Originally Posted by gofastsandman (Post 271380)

I like the way you think :D