Horses on a 20' center console

As in the above posts there are many variables(wt,waterline ,displacement etc). Mr Efficiecy's got a good formula.Just one big consideration.How fast do you NEED to go? I for one feel (in ocean anyway) that speeds past 27kts.are too fast.things happen quickly (ie. a partially submerged log) and usually you can't see a hazard such as this until it's too late to correct . Too often I have seen boats plowing along at 40+kts. oblivious to what they could hit-the waters are not a highway. I have had boats that can easily do 30+kts. but cruise at around 25kts. depending on sea conditions.For me an offshore trip say 30 nautical miles on a 23' boat is easily and safely done at around 25kts. but good to have the extra power when current/tide are against you.

 

You can't just look at the number of HP you need to go x speed in a specific boat and buy the motor with that number on the side. For instance: I have a 23' Seacraft center console that weighs about 5,000 lbs when fully loaded for a day of fishing/diving. Using the Croutch formula and a hull factor of 180, in order to go 30 MPH (my approx cruising speed) I require roughly 140 HP. Actual numbers: the boat is equipped with a 250 hp outboard, and 30 MPH is 4,000 rpms.

So, at cruising RPMs, the prop is getting about 140 HP to the water. Outboard engines are not rated to operate constant duty at WOT.

Now I'm building a 19' center console. it will weigh about 2,200 fully loaded with motor, people, gear. I want to be able to go roughly 30 MPH (26 knots) at cruise. Plug the info into the formula again and Crouch says I need 60 HP to do so. Except, I want to do it at crusing speed all day long. From my 23' boat, I'm cruising at 56% of available HP. So, that implies the 19' boat needs 107 HP. I'm putting on a 115.

 

Narcosis79, Did you calculate your own specific Crouch Constant or just use the 180 value I mentioned? Since your boat is much larger and 3-4 times heavier than mine, the proper Constant may well be somewhat different. You need known data from hulls similar to yours before you can derive a real value from which to make conclusions. Values range from below 150 to above 250.

 

Good point. As a check, I used the following: 5,000lbs, 250 hp (WOT), 42.5 MPH (actual per GPS). That gave me hull factor of 190.3. IF my actual weight is 5,000 lbs, and IF I'm actually getting 250 HP at the prop.

To see what a difference the hull form makes - the previous results would have been: 124 HP. That's a big difference from 140 HP at cruise. That would mean that 73% of RPM (4000/5500 RPM) only delivers 50% of HP. Possible?

wade