Seafari CG and stern-lifting props

[McGillicuddy]

Didn't want to derail the Sceptre 20 thread but why do we want stern lift on the Seafari 20?

Fa. Frank? Bushwacker? Seafari gurus? I'd like to hear more about this.

Thanks,
-Gillie

[Fr. Frank]

In a word, performance. Stern-lifting props just peform better with SeaCraft hulls, especially the Seafari and Tsunami/Sceptre.

I don't have the science and math to explain it. I do know that for it's size, the Seafari is a heavy boat. 1800 lb hull weight. Compare that to the '99 BUC Book weight of an '96 Trophy 192 (19'7") cuddy outboard model at 1680 lbs bare hull weight, or '96 SeaPro 186 (19'2") at 1412 lbs. (The new models of each only list the boats' weight as fully rigged including fuel weight)

The Seafari carries her weight with only a 7"6" beam and also has a fairly deep V entry, even though once on plane it has the full benefit of variable deadrise. Both of the above boats are 8' beam, with a more shallow V.

[gss036]

Fr. Frank suggested that I go to a 4 blade stern lifting prop on my 1989 23WA. That was the answer, raised the engine up 2 holes and installed the prop and it handles like a different boat and the mid-range economy is unbelievablely great. I posted some results w/a NorthStar fuel flow guage and everyone says impossible, but I just know what the guage is reading, even if it is 5-10% off on calibration. Go For IT.

[McGillicuddy]

Well that makes sense, The narrower beam certainly has impact on draft and general buoyancy.

I bought a Black Max 13.25 x 17 for a song and a test in patience. Just got it - bought it at Christmas... Anyway if I'm close to the RPM target and it pans out I think I'll seek out the PowerTech MQF series in the same configuration. If I get the RPMs right should I stay with the same pitch in stainless or would I want to drop down to a 15 or 16" pitch? Thanks for the advice. - McGill

[Fr. Frank]

Drop the pitch.

[Bigshrimpin]

eliminate stern squat
lower planing speeds
helps keep the bow heavy in chop.

[Bushwacker]

Fr. Frank said it well! About all I'd add is to consider going up in diameter as well as down in pitch. The 13 3/4" dia. x 15" pitch 3B SST was the optimum prop for my boat with the 115 on it. A 3B will probably be fine with a light motor, but I never tried a 4B on my 115. After I added bracket w/30" setback and repowered with a much heavier motor however, it was obvious I needed a 4B! With those changes, which probably shifted the CG aft by about a foot, judging by how much I had to shift axle on trailer to get adequate tongue weight, the 4B prop made a huge difference!

I just ordered a 4B PowerTech stainless prop for my rig; I had my 14 x 20" pitch Michigan Vortex 4B aluminum prop repitched to 18" for use as power prop for heavy load. Max load rpm was perfect at 5500-5600, but it cost me about 1 mpg compared to my 4B SS 14.125" dia x 20" pitch Michigan Apollo. I'm keeping the Apollo for light load, but I wanted a SS 4B for heavy loads to get some of the mpg back. I filled out their extensive worksheet and they recommended a SS 4B 15" dia x 17" pitch. I like the idea of larger diameter and less pitch as it should have less slippage and be a little more efficient. Spoke to Marcus at PowerTech, who really seems to know his stuff. He said any 4B prop will have more stern lift than a 3B prop, and that may be why my Apollo works so well, even though it seems to have some amount of rake to it and is listed as a bow lifting prop on their web site. The prop he recommended has similar blade geometry to OMC's old SST except for the extra blade, which he says has turned out to be an excellent prop; it certainly was for my 115! He said it will have more stern lift than the Apollo, although he said it will probably not match it on mpg. I also tried the Merc website, to see what they recommended as far as engine revs etc. For max economy they recommended a prop that would turn about 5200 rpm, which is about what the Apollo does with light load, but for heavy load they recommended a 5500 rpm prop.

The prop I'm getting has the same plastic hub system as the Michigan, similar to the new Merc props, so I can use existing splined adapter. Marcus said those plastic hubs will break pretty easily if you hit something, and they have a new patented hub design that cures that problem, but it's 2X the price! In addition to keeping the Apollo as a spare, I'll also carry a spare plastic insert, as they're only about $15. A friend of mine who runs a Merc with that style hub says he's heard that if you do break one, you can just rev the motor to 3000 rpm for a minute or so to get the hub real hot, then just shut it down for a minute or so and it will "weld" itself back together well enough that you can probably make it back home on a plane if you're real gentle with the throttle and just run at min planning speed!

Go to PowerTech's web site, fill out their VERY comprehensive worksheet and see what they come back with. I'll bet it's 15" pitch and a little more diameter than you've been running! Denny

[McGillicuddy]

I thought stern squat was eliminated by they porta-potty in the cabin

How quickly should I rise to plane in seconds, and at roughly what rpm or minimum mph should I get to and stay on plane.

I definitely concur on keeping the bow down in the chop and let the razor's edge cut away.

Thanks for the feedback. When I've asked some prop shops for a stern lifting prop in stainless that will run like the black max they all look at me funny and ask "are you sure you want stern lift, and why?" I tell them that's what the Seafari mentors tell me, now I have a few more reasons. And those coming from the guy with the fastest looking Ferrari, I mean Seafari of all, well sir, that's validation! And a new vocabulary entry: stern squat...that's like, when your really serious about taking a dump, right? Classic!

[McGillicuddy]

Thanks for the insight Denny, I actually just got the Black Max on Friday. Haven't had a chance to put it on yet. I bought it in 13.25 x 17 because it was super cheap and I thought it would be a cost effective test and a fine spare. I'll be replacing a 12.75 x 21 MWC Vortex(I think) that was only letting me turn 4500rpm. If it comes close to the desired rpm performance I'll order the 13.75 x 15 from PowerTech. Their MQF is apparently the Black Max equivalent in SST. If I could gain 1 mpg I'd be elated! That's another 30-40 miles of range!

Assuming the same geometry(or at least shape) what would be the expected performance trade-off on the bigger wheel with less pitch? Will I lose top speed but gain mpg with the bigger wheel or can such a correlation be made?

By the way I got the performance numbers you sent, apparently got lost in the mail. Pretty nice research. I'll use it as a guide to monitor my performance. Thanks. Never thought about how much slippage occurs at lower rpm. Definitely makes a strong case for a bigger wheel and more blades.

-Gillie

[Bushwacker]

Assuming the same geometry (or at least shape) what would be the expected performance trade-off on the bigger wheel with less pitch? Will I lose top speed but gain mpg with the bigger wheel or can such a correlation be made?
-Gillie

Good question Gillie - I have a 150 page book called the "Propeller Handbook" by naval architect Dave Gerr that says a rough general rule of thumb is that 1" of diameter is equivalent to 2-3" of pitch, and that 2" of pitch is worth about 450 rpm. That seemed to be about right on the MWC Vortex that I had repitched, which picked up about 500 rpm with a 2" pitch reduction. It also said that in general, larger diameter props were more efficient, unless you want to run over about 35-40 kts. It says 4 blade props theoretically lose a little efficiency due to the drag of the extra blade, but my experience is that they have other advantages that may offset that. All I know is that compared to Sandy's 3 blade Mirage 3+ that I tried, which is supposed to be a good prop, my 4B Apollo was about 3 mph faster, planned about 3 mph slower and was about .4 mpg better on optimum cruise fuel burn as measured by the I-Command gauges! It's still too much prop for my rig unless I'm running light. Would likely be a great prop for someone running a 175 or 200, maybe on a lighter boat. I've been trying to get Sandy to try it on his 20cc, but no luck so far! Denny