Help with proper WOT RPM and speed

[tmh]

New to this stuff but just bought a 1984 Mark Twain 19' Open Bow - great looking and somewhat unique boat imo. I'm learning here as I go so be patient. I now know I should learn the proper RPM range for my engine. Here are the specs.

Boat: 19' V Hull, open bow, about 2500lbs - a fairly heavy 19 footer BR from what I've seen. I don't know enough about the engine yet but here's what I think i know: I/O 1984 5.0L Mercruiser with alpha drive. (305 Chevy Small block i think) 2bbl. I was told it's 200HP and has a 1.65/1 gear ratio (rebuilt recently, used parts i assume).

Any educated guesses on what I should look for in WOT RPMs and Top Speed? It currently has a 19' pitch Aluminun 3-blade prop (haven't seen it's condition as it's been docked).

I'll be testing it a bit tomorrow (Thurs.) so i want to know what to look for. I already plan on adding some Smart Tabs to keep the bow down a bit when pulling skiiers and tubers. Also looking for lower comfortable cruising speeds. Top end less important, but I'd like to get close to where it "should be".

Thanks.

 

[Don S]

Re: Help with proper WOT RPM and speed

I would guess you have the 898 Mercruiser, which is a 305 Chevy small block. They were rated at 198 hp. (Pretty close to 200 )
The required WOT rpm from Mercruiser manual is 3800 to 4200 rpm. Normal load, properly trimmed.
If you are going to do a lot of tubing. Prop to the high side of the rpm range. Gets on step quicker.
Getting into the proper WOTrange is need to keep from overloading the engine (low WOT rpm) or not getting all you can from it (High WOT rpm).

 

[tmh]

Re: Help with proper WOT RPM and speed

Thanks much. Seems like a low RPM compared to others I've seen, but I think you're correct that it's an 898 Mercruiser so looks like that's the range. I'll let 'er out tomorrow to test - how should I have the trim set for WOT testing?

Any idea what the top speed range should be?

Any prop advice or should I ask that in the other forum? I may try one of the composite adjustable props just to experiment with pitch a bit then once I know more about what works, get a SS (4-blade?) for normal lake/bay use. Not too much to mess up the prop where I will be boating, but I'd keep the composite as an onboard spare!

 

[QC]

Re: Help with proper WOT RPM and speed

Thanks much. Seems like a low RPM compared to others I've seen, but I think you're correct that it's an 898 Mercruiser so looks like that's the range. I'll let 'er out tomorrow to test - how should I have the trim set for WOT testing?

Any idea what the top speed range should be?

Any prop advice or should I ask that in the other forum? I may try one of the composite adjustable props just to experiment with pitch a bit then once I know more about what works, get a SS (4-blade?) for normal lake/bay use. Not too much to mess up the prop where I will be boating, but I'd keep the composite as an onboard spare!

Work the trim up slowly until further up (out) slows her down, the prop starts to lose bite or you reach max trim limit. Don't forget to play with shifting weight around from forward to aft and visa versa. You will get the best top speed just before she starts to porpoise, so if you can't make her porpoise with up trim then bow is probably too heavy. If you can't give her any up trim without porpoising the bow is too light . . .

I am guessing right around 45 MPH if she isn't overloaded or waterlogged. How are you checking your speed?

Do you have a prop with known pitch now? That's where I would start before I bought anything. Your spare idea is good, but those composites are pretty slow at the top end I believe.

Good luck!!

 

[tmh]

Re: Help with proper WOT RPM and speed

Thanks a ton, Don. Great info for me to use today. I'm finally "getting it".

 

[Mahoney]

Re: Help with proper WOT RPM and speed

Apples to Apples comparisons should definitely be made on similar water conditions as well as similar atmospheric conditions.

Not sure,and I am betting many members can attest, doing a run on a nice cool 55 degree morning is going to give you much better results than the same run in the middle of a heatwave on the lake at 95 degrees.

 

[calwldlif]

Re: Help with proper WOT RPM and speed

44 mph with 3% slip at 4200 rpm with 19 prop
would be the best at your current prop.
http://www.rbbi.com/folders/prop/propcalc.htm

 

[tmh]

Re: Help with proper WOT RPM and speed

OK, thanks. Great site for calculating this stuff, btw.

But what IS "slip" and what effects it? Can I control it at all?

Thx.

 

[Limited-Time]

Re: Help with proper WOT RPM and speed

Slip is the prop spinning in the water without generating forward motion. It is effected by hull design, drag and weight to mention a few things.