tach, WOT and prop

[tmcalavy]

Finally got the tiny tach installed on my Merc 500. Took the boat out today and was able to make 4830 to 4880 rpms at WOT, cruising along at 23 mph. Just me (275 lbs.), 4 gallons of gas, and a 51 OMC 5 hp kicker (50 lbs.) motor in the boat...not much of a load. The boat is a 72 Ebbtide 15 foot trihull runabout, with a 10.25 dia. X 13 pitch prop. I bought this boat at about 800 ft. above sea level, but it runs here at about 3,500 ft. above sea level...and it hasn't been turned for altitude. Now I'm not a speed demon, but can I adjust the prop to put this puppy at about 4800 to 5000 WOT and an even 25 to 27 mph or faster top end?? I hesitate to tune the carbs for altitude since I do road/boat trips to lakes at lower altitudes regularly. Strategizing on the prop will give me something to do while the lower unit is in for new seals...replaced the water pump and impeller before today's run but the case still had vanilla milkshake in it when I got home and checked. Any opinions welcome.

 

[JB]

Re: tach, WOT and prop

Howdy, Tim.<br /><br />I just put your boat into my calculator at a gross weight of 2000 on the water and came up with a theoretical WOT of 28mph.<br /><br />That assumes you have the perfect prop, perfect trim and no extra drag in the hull.<br /><br />You didn't mention whether you have power trim. If so, how do you optimize it? Also, is your mph coming from a water instrument or GPS?<br /><br />I would work on optimizing the performance of what you have, rather than trying to get a few extra mph with a new prop. If you don't have power trim, try setting the engine out a notch or two and see what happens.<br /><br />Good luck.

 

[tmcalavy]

Re: tach, WOT and prop

Thanks JB,<br />I have manual tilt-trim. I run the boat with the engine bracket pin in 3rd hole out from transom. It has a doel-fin so it gets on plane well, just doesn't seem to have much top end. I plan to compound and wax the lower hull while the lower unit is in for seals. That may help a little. Then I think I'll rebuild the carbs, if I can find a schemo to help me get to them. Then I'll check rpm-WOT-mph again. My mph reading if off the speedo that came on this boat, so I don't know its accuracy or history. Just know the Merc tops out around 4800 to 4900....when top end is rated at 4800 to 5500 (its an early 70s engine). Yesterday was the fastest I've seen it go...23 mph seems a tad sluggish with very little load. I'm not looking for more speed necessarily, but just want to know why it won't hit the 31-35 mph listed in my service manual prop charts. Any advice/input is welcome.

 

[steelespike]

Re: tach, WOT and prop

Tim,Before starting any repairs get yourself a manual.The library has manuals for reference<br /> or try NAPA,iboats.com,amazon.com,ebay,etc.<br /> If your motor starts good,idles and responds to the throttle leave well enough alone!Check for<br />throttle opening,syncronization, timing,point, and plug gap.Also check your compression.<br />Grubbs Marine at: www.oldmercs.com has reprint owners and ser. manuals for your motor.<br />I would sure guess at 3500ft your power is reduced<br />some.

 

[wilde1j]

Re: tach, WOT and prop

Tim, you know your motor is about a 43 HP motor at an elevation of 3500 ft? Naturally aspirated motors lose about 4% of nominal power per 1000 ft of altitude above sea level. You didn't say how it performs at sea level, but it's certainly going to be faster. High speed jet replacement is probably not worthwhile at your altitude, especially if you also boat at sea level. If you really need more speed, a bigger motor is the reasonable answer.<br />Jim

 

[Lark40]

Re: tach, WOT and prop

You really can't trust a water pressure speedometer. I recently bought a boat that the owner swore would do 52 mph. According to his factory water pressure speedometer, that is what it would show. I knew that was way high.<br /><br />Used two GPS to determine actual speed - both showed WOT doing 39 mph.<br /><br />Would recommend you borrow a handheld GPS to get good numbers, and then work from there.

 

[tmcalavy]

Re: tach, WOT and prop

Yeah, I'm wondering about the old speedo. I'll have to check mph with a GPS unit or just ask someone to run with me and note their speedo when I get to WOT. <br />Hey Steelespike. I have 2, count 'em 2 manuals, and I ain't gonna just leave it be. I want to get this boat running the way I want...the way it should run according to factory specs, with some slack for age, etc. I'm not looking for speed, just trying to find out whether I have the wrong prop or maybe need some carb work. So lighten up on the "buy a manual" sermon, man. Change the station until they play another song you like. Life is too short to obsess.<br />Thanks for all the replies. Any other ideas welcome on this subject.

 

[MGuckin]

Re: tach, WOT and prop

Tim,<br />Check out these 2 posts.<br /> Determining X Factor Adjustments <br /> Theoretical MPH<br />Keep in mind the altitude variable and that your motor is not prop hp rated.