Prop for 23 SeaCraft with transom-mounted Merc 250 EFI
 

Hello,

Does anyone have any suggestions for this setup? The SeaCraft is a 1978, and the Merc 250 EFI Offshore is from 1996.

I'm currently running a 17 pitch Mirage Plus, and am only getting 5,400 rpm and 46/47 mph with an empty boat.

Another thing is the prop blows out on turns, and the engine still needs to be raised a hole or two. Thoughts?

What's your goal?


I would get the engine height set correctly first. It sounds like you are close. The mirage plus is a large diameter lower rake prop (15.5") and it does blowout pretty easily.


Rev4 17p will hold a lot better, but you might lose a few RPM's over the mirage plus 17p.

I personally would try an 18p Enertia (non Eco) which are faster than the mirage plus and hold a lot better.

19p Highfive is a very small diameter prop that's good in the low to mid range. That prop can ride high and won't blow out.



I ran a 225hp 3.0L carb Mariner with a 17p Rev4 for a week (before I sold the engine). Running light I could get into the high 40's trimmed up.

http://www.wellcraftv20.com/gallery/...m204/225_G.jpg
http://www.wellcraftv20.com/gallery/...00_1.sized.jpg


On the current engine 150hp Merc 4 stroke . . . I either run a 15p Enertia or 17p Highfive (depending on load). http://www.wellcraftv20.com/gallery/...811_035022.jpg

Appreciate the reply! I'm shooting for a prop that gets me to at least 5,600rpm, and doesn't blow out. Good hole shot is a strong plus, as well as high cruising and top end speed.

What rpms were you turning on the 225 with the 17p rev4?

If I remember correctly it was 5800rpms. That 3.0L engine drank fuel. I could believe how much it used. I put a 2.4L frankenmerc back on after that one until I repowered with the 150.

Rev 4 is the direction I would go.

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?

Prop Gear Ratios
 

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?

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

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)

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.

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?

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

I like the way you think :D

OK you got me to bite on this one . I have thought about the design of T tops for a while I having had mine custom-made it was always intrigued about how the angle of the top acted knowing full well that if it angled down it would be driving the boat into the water and too much at the front would act like a Parachute. The idea of designing it like a wing is intriguing! again the angle in relationship to a boat/deck that is constantly changing going up and down a wave is of concern and debate in my mind. I Would think that it would continue to supply lift to the hull And then again we would need to consider the placement as too far forward or too far aft could be the detrimental

Well I raised the motor a hole and rpms increased to 5,600, but speed stayed around the same. Still running the mirage plus 17 pitch.

Seeing as it's frustrating the prop will ventilate when accelerating in a straight line (flat conditions), my priorities are primarily the prop that has the most bite. Also since I have no trim tabs, the prop with the most stern lift would be desirable.

Re-reading everyone's replies, seems like the REV4 is the ticket. Any objections? :D

Since I'm only seeing 5,600rpm with the Mirage Plus, does it make sense to go with a 15 pitch Rev4? I'm concerned about the rpm loss

Lots to consider.

Carl designed and ran a flying wing hull.

I think the ocean being water and the air being easily compressed may
present too many variables to be done safely. It would be difficult
to fish around a tail section as well.

The sailing cup racers are using foils with wind and underwater.
Lots of carbon fiber and ridiculous forces.

I love my Rev4. Never went back to a mirage plus. same issue as you cavitated with M+ only on turns. 250 Verado had a17P R4, and was just barely getting WOT needed. When I sourced a 275 ECU it was perfect peachy with a 17P R4 so you are probably right its on the edge of a 17P or 15P R4. Turns out I could have gone much higher with the R4....never did raise from 2008- till this season with a one year old used 300 Verado i snagged.

No idea why or where read (tht? and propgod?), but the 15P Rev4 had some reason for not being advisable desirable.

Raise you engine even more and get a R4 17P....I suppose you could go silly $$ and get a R4 XP 17P....

Right now, 300 V8, R4, running a 19P R4 heavy load and barely if even need to trim up, hitting 6K which is max end of WOT range. DO have tabs. I had the marina raise the engine so high I was sure it would need to go back down. I was wrong. Those R4s can really run high. I am debating a 20P or even 21P R4 for this summer.

GL, def some art and voodo to propping, heights.

As an update, I tried the REV4 17 pitch and lost 6mph and 500rpm. It certainly held a lot better, but the loss in performance is too much. I'm thinking of trying Bigshrimpin's suggestion and going with an Enertia. I see he recommends an 18 pitch; does the enertia turn more rpm than the Mirage plus? I wouldn't mind gaining a few rpm bringing it up to 5,800 with a light load if that's the case