Prop for 23 SeaCraft with transom-mounted Merc 250 EFI

[rdoaner123]

Originally Posted by Capt Terry

There may be a difference in gear ratios like there was between Bushwacker’s 150 ETEC and my 150 HO ETEC, both on 20’ Seafaris. I dont remember the details but his liked the Cyclone better than mine, so he bought it. I had better low speed control at 17 mph and achieved the desired WOT RPM with a 17p High Five.

Makes sense... which begs the question (for Bigshrimpin): what year was your Merc 225?

[rdoaner123]

Originally Posted by rdoaner123

Makes sense... which begs the question (for Bigshrimpin): what year was your Merc 225?

Actually I looked through my collection of Mercury brochures, and the first year of the 3 liter the 225 carb (1994 year) had a ratio of 1.64 to 1. After that, all of them (even the ones being made now for overseas users) are 1.75 to 1. So unless someone replaced my LU with the old one, it's not possible that Bigshrimpin's outboard turned a lower gear ratio than mine. If anything, it's more likely that he turned a higher gear ratio

[Bigshrimpin]

Originally Posted by rdoaner123

How can it be that your 225hp motor turned a prop at 5800 rpm when my 250 can only turn and easier-to-turn prop at 5,400 rpm? Is there something wrong with my motor?

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

[rdoaner123]

Originally Posted by Bigshrimpin

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?

[Bigshrimpin]

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/

[77SceptreOB]

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)

[Bigshrimpin]

Originally Posted by rdoaner123

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.

[77SceptreOB]

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

[rdoaner123]

Originally Posted by Bigshrimpin

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.

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.

[gofastsandman]

Originally Posted by Bigshrimpin

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

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