what is the minimum HP I would need to power a 21 foot aluminum boat?

[jimmwaller]

I am thinking about buying a 21 foot starcraft chieftain, but it doesn't have an outboard and I don't know that I want to drop thousands on a new outboard. I spend most of my time in a bay where they chop only gets to like 1 foot max. I usually use a 17ft lone star riviera with a 20hp and that's plenty of power for me (I don't even go more than maybe a mile or two out, so I don't need a full speedboat outboard).

For reference, I go about 15 mph with the lonestar, and I don't need to go much faster than that on the chieftain. But hte chieftain is much heavier, it just has more metal, a deeper hull, and a cuddy, etc. so I'm guessing that even like a 30 won't cut it.

I have an extra 40-- is this too small? I also have a friend who will sell me a 60 for super cheap. If even that's too small, I might just pass on this one as I can't afford the repower

thanks for your input!

 

[ondarvr]

The starting point is what's the hull rated for (if you don't have that info someone here will).

You are going to need something in the 90hp range for even adequate performance.

 

[b.gagnon]

I would say 90hp Minimum...... I had a 90hp on a 17' Seanymph and it only did 35mph (Not loaded) and trimmed sky high!
115 would probably be a great fit and it won't struggle to push the boat. A 60hp will have to work very hard and be lugging (Not good for the life of a motor).

 

[strokendiesel002]

I have a 21 ft 'toon with an evinrude 60hp bigfoot I think they call it. I get about 21mph with a very light load. Check for prop selections available for your 60 in question, and if there is a large selection, go for it. I assume he's including the rigging?

The most important thing in my opinion, is that you can put a large enough prop on it to control the boat in foul weather given the usage you described. High speed isn't important for everyone, and unless that boat is really decked out/ water logged, it really shouldn't be all that heavy.

Those are my thoughts, anyone else? He has the rest of his time with the boat to repower to a 90/115. 60 isn't going to win any races, but should be able to safely maneuver it. Patfromny you have an 18' deep v with a 40, correct? It performs adequately enough from what I've read (great work, BTW) so I think an extra 20hp would compensate for the extra 3'

If I'm way off in my thoughts, please let me know and I'll edit/ delete.

Good luck! Hoping for pictures if you get her!

Jimmy

 

[tomhath]

I suppose you could try the 40 and see how close you are, but you'll likely need a lower pitch prop to get on plane.

It's not clear to me that a smaller motor will be much cheaper. It will probably use about the same amount of fuel and will work the motor much harder.

 

[BWR1953]

What year is the Chieftain?

​My 1976 outboard Chief is 18' and is rated for 150 max.

 

[mr 88]

Slap the 40 on and then the 60 to see how it performs.As noted you probably will have to go with a lower pitched prop to keep from lugging the engine, that's where the tach comes into play. Maybe try putting the 40 prop on the 60 if it fits and is a lower pitched one. Normally the rule of thumb is a minimum of 65% of the rated HP for it to perform decent, but your not after that,just trying to use what you have. I have a 14' aluminum rated for 40 hp and just have a 6hp 4 stroke yammi on it and it gets up and planes with just me on board,but its a dock boat that I use to get around in the harbor when the big boat is too cumbersome to do that for various reasons.

 

[Patfromny]

My boat is a 16 ft with a 40 on her strokendiesel002. (Thanks for stopping by the thread)The 40 does ok at right around 30ish but I am having trouble finding a prop to get her in the proper rpm range. I blame Evinrude for their lack of depth in the prop department. Most think the problem is a motor too far away from the max for the boat (90). I think if they need to add a few props to fill in the holes. What year/make engines are we talking about Jimmwaller?

My advice Jimm, is to buy the 60hp. More hp is your friend. It sounds like you just want to get out on the water but the 40 might not even get you home in a sudden storm. My brother has an etec 40 on a 19-21 foot pontoon and it will go 21 with just him in it. Three people and it goes 15. Sell the 40 and stow the cash away...drive the boat with the 60 and see how you like it. If it isn't doing it for you, sell the 60 and combine the proceeds from the two engines to purchase a 90 or above. Don't pass on a boat because you can't afford an engine. I had this new to me etec land in my lap for 1100 rigged and installed. There are deals out there if you're patient.

 

[ondarvr]

I'm not sure where this pontoon stuff is coming from, it's a totally different application than what the OP is asking about.

​If you have a motor, put it on see what it does, but DO NOT buy a motor that is far below the HP rating just to see if it "might" work, get the right size motor after seeing what it's rated for. You don't need to max it out, but don't go significantly under it either, and rule of thumb is 75% of the rated HP, but like every other rule of thumb, it's just a general suggestion, every boat will respond differently to HP.

 

[Patfromny]

I'd put the biggest engine you could afford on it was my point. That seems to be his buds 60. The pontoon ref was just meant as an example of speed on the same size boat. I realize they are different but not 20 MPH different. For sure, the 60mwill do better than the 40 would.

 

[ondarvr]

A pontoon cannot be compared to a 21' Chieftain in any way, pontoons don't rely on planing, they can perform very well with even a 9.9, a normal style hull will reach hull speed without much power, and until you have enough HP to actually get on plan the increased HP doesn't increase your speed much at all. In this case if the 60 can't get it on plane, then it won't go all that much faster than with the 20, so there's no reason to waste the money on something that has only slight, and that's very slight, chance of working.

 

[Patfromny]

Seems as though my usefulness here has run its course. Good luck. Hope you go for the Cheif.

 

[roscoe]

I used to have an 18' with a 65 hp, took forever to climb out of the hole and yes it was propped right, actually high, for wot rpm.

You are going to need to get her on plane in order to break 6 mph.
The 40 will never do it.
A modern large displacement 75 might, but I doubt it.
90 hp minimum is my suggestion to get it on plane, and it might run 22-25 mph.

 

[jimmbo]

I have an extra 40-- is this too small? I also have a friend who will sell me a 60 for super cheap. If even that's too small, I might just pass on this one as I can't afford the repower thanks for your input!

What year is the 40hp?

 

[fhhuber]

Depends on performance desired... and how heavily loaded the boat is.

You can troll on a 5 hp. But might have some issues dealing with wind and/or currents.
40 will get you moving along pretty decently but may not get you on plane. Better than nothing and gets you out fishing...

You'll want 80% to 100% of what the hull is rated to handle.

 

[Scott Danforth]

if you never ever want to get on plane and you just want to putt along, the 40hp will be fine as ondarvr pointed out, hull speed for a 21' boat is 6 mph. if your ever in current over 6mph your going to be going backwards unless you can get that puppy on plane

with a small pitch prop, the 60 could maybe get the boat on plane, then your going to over-rev at WOT.

I personally believe that 130hp in a 21' is under-powered. I believe the boat is rated for 170

a 20hp on the lonestar performance-wise would be like a 100hp on the chieftan

 

[fishin98]

I would think 90HP minimum...There are many nice 90HP-150HP out there to be had new and used. I had a 2000 Bayliner 1802 Capri Cuddy with a 2000 Mercury 3 cyl 2str 90HP..Pushed the boat with 4 people around quite well. super reliable,easy on fuel and to maintain. Get the boat,then worry about a motor...shop around for a current good motor at a great deal. Messing around with smaller motors is just going to add to your frustration.

 

[ezmobee]

90 minimum. My buddy had a 90 on a 21 Chief. It planed it well but top speed was low-mid 20's and you were pretty much WOT all the time

 

[redneck joe]

Sell your 20 and 40 and see if that and your cash can get you a JohnnyRude 4 cyl of some sort; I see them for $1000-2000 all the time.

Whatever you do, if the Chief is a good deal - buy it. You won't be sorry.

What part of the country are you in?

 

[Sprig]

I have an 18' Alumaweld heavy gauge aluminum boat rated for 115 hp. I therefore think your boat is rated for a lot more. I don't get why anyone would want minimal and under powered horse power. It just doesn't make sense. You certainly don't have to use all the power but it's good to have it. You should power boats to their max or close to it. If you want to go 3 mph you still can but when you need the power and speed you have it with adequate hp. Resale of a grossly underpowered boat will be difficult at best.