Slow boat?

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Had my boat out today, 1989 Larson 17' fiberglass runabout with a 3.0 L Mercruiser stern drive. Top speed with me alone was 32 mph...seems a little slow for a 130hp engine. Thoughts?
 
What is the history of the boat? Did you get the engine to WOT?

Fresh gas? Prop in good shape? Boat bottom clean? Bilge empty of water?

How did you measure speed? GPS or speedo?

Trim tabs set properly? Engine trim?
 
All of the above, plus engine timing curve. Why folks go out on the water with unknown OEM timing curves and don't know if the engine is lazy, or on the edge is a little surprising ...

Any pings on the current gas (last season...) while accelerating out of the hole?
 
Originally Posted By: Donald
What is the history of the boat? Did you get the engine to WOT?

Fresh gas? Prop in good shape? Boat bottom clean? Bilge empty of water?

How did you measure speed? GPS or speedo?

Trim tabs set properly? Engine trim?


Whoa, let's see...
History -- had 1 year IDK before that
WOT -- I think so
Fresh gas yes
Prop -- a little ratty
Bottom -- clean
No extra water on board
Speed measured by fish locator and speedo
Motor was trimmed up
Trim tabs?
 
OEM timing curves??? Huh? I get that you're formerly in marine engineering but I'm just a 10 use a summer family guy!
 
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I'd expect about 40 with that boat, maybe a touch more.

Do check for WOT. It's easy- engine off, remove the cover and air cleaner, move the throttle level all the way open and check your butterfly(s). Adjust as necessary.

Having the correct prop in good shape makes a HUGE difference.

Do check your tune up, point dwell and yes, timing.
 
How's the exhaust too? Sometimes old transfer hoses to the out-drive can come apart internally and partially block flow... If they've been on there 10 years, I'd just do new hoses. They don't need to be Merc, they can usually be Gates Green Stripe in the appropriate size and with with new all SS clamps. I usually double clamp top and bottom...

OK, for speed or acceleration - the prop is the biggest thing assuming the motor is optimal as checks against all the above listed items... OH, and you might want to check the coolant op temp. A cold motor will not make the power either...

So as the boat comes up and the motor starts to spin it all, the prop edges get more and more important. As RPM increases, prop condition like rotational drag goes up with the square of blade speed. That's why guys chasing speed and fuel efficiency are freaks about prop condition.

I know it's a family boat for some summer fun and all. But it's like tires on a performance car. It will not perform w/o the best prop for the load. The best prop will usually be a three blade calc'd and checked with the family on board and full fuel, cooler with some beer, etc. It should load a well tuned motor to say 100 RPM from max efficiency point. I think yours is like 4,700 or so. So all family loaded on plane with no headwind, maybe 4,600 or there abouts.

Once it's just you or you and wife or daughter, it will pick up that last 100 RPM and be a bit faster.

The most efficient props are the razor sharp SS props with thin blades and a speed cup. That cupped edge does nothing down low, but when you are on the last bit of motor and out on the lite side of loaded, it can be worth 2~3 mph.

So do you have a Tach? To really figure this out, you need a well tuned motor and you need to see the max RPM under load...

Take a small file (half round & flat) and dress the edges of the existing prop to get rid of bumps and burr's. If it has a real bend in the edge, Hammer it back with a backing block (piece of flat Oak with rag to hep soften the shock, and then file flat.

Sand it down and paint it with some engine paint. Not for slippery, but so we can see what happens after the next run.

If there are any cracked blades - forget it. Buy another prop, or send it in to be welded and re-pitched... Then start the hunt for the optimum
smile.gif


you get it tuned right and propped right and it will drive well and be more fun
smile.gif
 
Rumblejam's link to Merc shows 35mph max speed. IDK, will try the other suggestions to eek out a few more MPH. RPM at max speed is about 5k. Thx for the great input fellas.
 
4800 is max for that engine. That Merc test is swinging a 21 pitch prop and 200 rpm shy of max. It appears to be full fuel but does not state how many people on board.

If your prop is less than perfect it is costing you mph, rpm, and more fuel.
 
It's amazing what a ratty prop will do to performance, it's the equivalvent to the tires on your car, the only place your boat can gain traction and if it isn't in good shape your losing traction. It's like trying to drive in the snow on bald street tires instead of having a fresh set of snow tires, it'll work but it's not going to work well.
 
My 18' Mako has a heavy top deck on the hull and does 10 mph over that w/ 115 HP Opti (SST Merc prop)
Also have a flat bottom fiberglass 16' tunnel w/ F60 & does 32 mph ... Yeah, your motor needs TLC ...
 
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