new prop selection help for motor that is too big for Hunter 23

[scott hall]

I have a hunter 23 with a 9.9HP Merc kicker. Max theoretical hull speed for boat is 6.5 knots. From what I have read a 4 to 5 HP is all I need. Going to take a trip on Cumberland river a 100 miles downstream than a 100 miles upstream against a possible 3 knot current. Desire to get a extra prop. What type prop should I get. I believe mine is currently a 8 pitch. A prop shop said to get a 6.5 pitch. But the lower the pitch the more fuel it uses if I am correct. Not sure if I should go up or down on pitch size or if a 4 blade instead of my current 3 blade has any benefits,

Any help appreciated.

Thanks Scott

 

[dlochner]

Scott, I can appreciate your ambition, however, your hull speed calculation may be a little high. There are several formulas out there, from 1.22 to 1.44 times the square root of the waterline length. At the high end the calculated speed is about 6.37 knots at the low end it is 5.39 knots. My suggestion is get the boat out on a flat calm, no current body of water and see how fast it can really go. Realistically the best you'll probably do is in the mid to high 5 knot range.

Even if you get to 6 knots of boat speed, speed over ground will be around 3 knots, that is somewhere around 30 hours of motoring up river. That is a long time listening to the drone of an small outboard motor.

Why not go 200 miles down stream and have someone drive down with the trailer and pick you up?

 

[Will Gilmore]

Lots of people confuse the motoring against current with the need for more speed. Most sailboats are displacement hulls and pushing a boat beyond hull speed is harder than pushing a boat up to hull speed. Hull speed is a measurement through the water not over the ground, so if you can't improve your speed through the water in no current, trying for more speed against a current isn't going to work any better. If you are fighting wind and chop, you may want more power so you can maintain hull speed. 9.9 hp should give you plenty of power to reach hull speed and give you a little extra to fight opposing winds. I would go with a prop that improve thrust over speed. Then, to save fuel, throttle down so you are just making hull speed and don't try to go faster.

- Will (Dragonfly)

 

[MitchM]

i think i'd do a test run of going 3 miles up the river against the current to see if this OB will drive the boat like you want... putting a larger prop on some OBs is a bad idea. what does the manufacturer of the OB say about trying to get more power by changing the prop ? ?

 

[Johann]

You could use a high thrust (lower pitched) prop in your situation, but it might not get you any extra speed. Certainly if you got a higher pitched prop you would lose out on efficiency, and maybe even speed. HP is torque x RPM, so with the standard or high pitched prop you may not get to rated RPM at WOT, and thus will not be getting full power from your motor. And you will be using more gas, not less for that power.

A high thrust prop should allow you to get to rated RPM at WOT, and therefore rated HP. But you still won’t go much faster (if at all) than your hull speed. As Will said though, you will have power to fight headwinds or chop that might otherwise slow you down.

I would go with the prop shop recommendation.

 

[DougM]

Scott, I can appreciate your ambition, however, your hull speed calculation may be a little high. There are several formulas out there, from 1.22 to 1.44 times the square root of the waterline length.

For what its worth, split the difference. Hull speed in knots 1.34 x sqrt LWL.
“It has to be true, I read it on the internet”. In any event , its a theoretical number.

 

[dlochner]

For what its worth, split the difference. Hull speed in knots 1.34 x LWL.
“It has to be true, I read it on the internet”. In any event , its a theoretical number.

What? you mean the internet has fake information???

We also haven't addressed the Knots vs. MPH. Those 2 ideas get confused. He may well have a theoretical hull speed of 6.5 miles per hour, however, that is about 20% slower than 6.5 nautical miles per hour. A nautical mile is almost 20% longer than a statue mile.

 

[JCall]

https://forums.sailboatowners.com/i...r-a-mac-classic-d-s.113962/page-4#post-689113

This is a long thread but well worth reading. Walt and Sumner provide good information based on real personal experience. The Merc 9.9 is a good motor, an extra prop is a good choice to make before you go. No mater what motor or prop you have, you will be going against the current heading up river. Boat speed - current speed = ground speed. More time to enjoy the scenery!