Prop selection query

[nodima]

I am looking to change props on my Boat, and am looking for some advice.

The boat is a 2004 16' Lund Alaskan Tiller
Rated for up to a 50hp motor

Motor is a Mercury 25 Bigfoot 4stroke 2005
Gear ratio is 2.42; according to the motor it should max at 5750RPM

Current prop is a 11.25x10 3 blade.

I installed a tiny-tach last night, and took her out on a local lake to determine what my RPMs at WOT are. The boat had 1/2 tank of fuel (~10 gals), me 180lbs and ~50lbs of misc gear.

Idle 900RPM
WOT 3010RPM @ 19MPH

These numbers do not make sense to me based on some of the calculations I have done with various prop calculators. The speed seems about right compared to my old 16' boat with a 25 but the rpms seem awful low.

Questions: Could the tach be reading 1/2 of what it should at WOT? How would this be explained if it is reading correctly at idle?

I realize the boat is underpowered with only the 25, but due to regulations on one body of water I fish, a larger motor is not an option.

As I add additional weight, especially in the forward seat, the performance takes a hit. I don't particularly care about top end speed, but keeping it close to 20 is great.

From what I have found online, for this engine, as the pitch lowers, the diameter increases, to 12.5". A 12.5" prop will only give me 1/4" clearance between its tip and the ventilation plate. Should this concern me?

Thoughts?

Thanks

Nodima

 

[steelespike]

Re: Prop selection query

I think I read somewhere that sometimes the tiny tach will pick up stray pulses.You may need to read your instruction carefully to be sure you have followed any precautions.Does the motor seem like it is only turning 3,000
rpm?Thats barely cruising speed.How is the hole shot?
I would think 10 in pitch would put the motor in its operation range.
Is the tach for a (4 stroke). 4 stroke has half as many pulses
And it could result in 3,000 rpm when it is actually closer to 6,000.
If it was running 6,000 it would sound like it. Very busy!Of course way beyond reccomended.

 

[nodima]

Re: Prop selection query

You nailed it. I spoke with a rep from Tiny Tach, who said the same thing, and to fix it, to either do 7 wraps around the one plug, or 3 and 3. The 3 and 3 seems to have worked like a charm.

With just me, a 1/4 tank of fuel ~6 gals, and a minor amount of gear, and a 10.88x11 prop, the motor turned 5800 rpm, at 20.5 kts. I tried several of the trim positions while I was testing as well.

Next is to test with a second person on board. And try again with the 11.25x10 prop.

Thanks.