[Lone Ranger]

In an earlier post I stated that boat speed was a function of engine horsepower and if you put a large horsepower engine on a boat, the speed increase was a function of the horsepower ratio to the cube root (all other things considered equal such as boat weight, boat load, gallons of gas, etc. Engine weight would increase with the larger motor). One Guest disagreed with my thinking. Here's the formula I had in mind:

Top Speed 2 = Top speed 1 x (HP2/HP1)^0.3

Meaning the new speed is calculated by taking the ratio of the new HP divided by the old HP to the cube root times the old top speed.

I don't think this would hold up for cars or motor cycles, just boats.

What do you think is the correct answer? Maybe for planing hulls I should be using the square root? What do you think?

 

[fsh2win]

dude, put the pipe down... youve had enough! ha ha LR, you did leave out gear ratio for the outboards, now back to the drawing board for you. later, kk

 

[Boat Cop]

Tis is way toooooo deep with all the goezintas and stuff. Big motor speed, little motor no speed.

as for your rithmatic i dont know about that. G&F and us have the same identicle boat to the screw. they have a 225 saltwater on theirs and it does 51 on a good ripple day. ours has a 250 saltwater and it does 61 on the same day and ours is quicker out of the hole.

 

[gcbaitman]

gcbaitman

What happens if your motor is bored out? what happens if yoo have a v-hull semi deep v-hull ? what about outside temp and sea level? hhmmmmmm.......... j/k

 

[Delw]

Lone Ranger the Guest that disagreed is correct.

Its by rpm and Pitch.

the more horse power you get the more pitched prop you can turn makeing your boat go faster.

The only thing RPM does is allow you to turn a bigger prop...



Delw



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[Bayou]

Anyone get a headache from this thread? :?

 

[Lone Ranger]

Good input guys. Let's just cancel this thread and go fishing! Del I had not thought about the bigger engine/larger pitch prop thing, that could get overlooked. Cancel this thread and let's go catch some fish instead!! :lol:

 

[Spidey]

You forgot the load factor. I used to plain off right away and can cruise about 50mph at 5 grand rpm with a 90 hp merc. Toad has gained weight and is less aerodynamic than he was before. He also brings 9 rods and reels, 3 worm bags, 1 overstuffed tackle box not to mention his 20 quart lunch cooler. I would think you would need a computer program in order to determine anything. Here is the big question, how much weight would a 210 pound toad need to lose in order to have the boat plane off in half the distance on a calm day?

 

[conquistador]

Also don't forget the motor placement as can be achieved by a jakeplate.

I'm no expert, just got my first jakeplate. But I know that if I raise my motor, getting more of it out of the water, reduces drag and allows my boat to achieve an higher RPM at the top end.

I'm hitting my max RPM that is desired for a 225 Opti. and still have more throttle left. As Delw says, I need to increase my prop pitch which will increase top end but hurt hole shot. But I still have the jackplate that I can further adjust.

Of course it is winter and the motor will run better. Not sure what happens in summer.

 

[MultipleMigs]

My apologies for not identifying my post, Lone Ranger. It was me that made the blurp about your theory on boat speed vs. HP 'not holding water. It really boils down to several things. But assuming your holding things equal, i.e. ambient temp, elevation, etc., it really is a function of RPMs, Prop Pitch, Gear Ratio, and Prop Slip. Now, I won't bore you w/ details, but you can refer to the formula I posted recently to a response for ideal prop selection. Do a search using 'prop' or if you need info on this I can send a PM. These guys' heads are hurting!!! :shock:

 

[Lone Ranger]

MultipleMigs, no problem. After reading what Boat Cop said about going from 225 hp and 51 mph to 250 hp and 61 mph, I realized that there was a lot more to it than just RPMs. Thanks for your post. Yes, please do send me the formula in a PM. 8)