Use maximun HP rating?

[hthomas49]

I'm down sizing from large Triton to 17-18 ft aluminum. It seems like most used boats have engines below the max allowed. If a boat id rated for 70hp but has a 50 how badly will this effect the performance? Or 75 rated with 60?

 

[southkogs]

Welcome aboard:

The general rule of thumb is your minimum HP should be no less than 75% of the max rating. So, on a boat rated for a 70HP, 50HP would be as low as you would want to go. How badly will it effect the performance is really going to depend on the hull, and what your expectations are. I've had a boat that was severely underpowered, but for what I was doing with it things worked out fine.

50 would likely get you on plane with a light load, but wouldn't get you there fast. (as a guess)

 

[Watermann]

There's no way an 18' AL fishing or runabout boat would be rated as low as 70 HP. I have an 18'r that is rated at 140 and a 16'r rated for 70. On an 18' boat I would not go under a 90 but a 115 would be best all around.

 

[WIMUSKY]

What's the boat? Utility? What does the plate say for max HP?

 

[shrew]

There's no way an 18' AL fishing or runabout boat would be rated as low as 70 HP. I have an 18'r that is rated at 140 and a 16'r rated for 70. On an 18' boat I would not go under a 90 but a 115 would be best all around.

An Aluminum Fishing/Runabout like a Starcraft and an Aluminum utility boat are two very different things. Advising that people well exceed the capacity plate is simply irresponsible.

 

[Watermann]

An Aluminum Fishing/Runabout like a Starcraft and an Aluminum utility boat are two very different things. Advising that people well exceed the capacity plate is simply irresponsible.

Seriously shrew? I usually ignore posts like yours... and probably should again.

hthomas I would also mention that you'll find a lot of people when they buy their entry level boats from dealers will buy them with a smaller motor due to the lower cost and is most likely one of the reasons so many sell the boat off as it hasn't lived up to their expectations being under powered. I would suggest not dropping coin on something that isn't close to being at it's max HP especially after having a Triton as I think you would be unhappy with it's performance.

 

[WIMUSKY]

Seriously shrew? I usually ignore posts like yours... and probably should again.

I agree with shrew. How can anyone blindly make a hp recommendation when no-one knows what type of boat the OP has?

 

[Sea Rider]

Max HP transom rated is for Max passenger/load capacity stated on tech plate, if you go boating constantly with less passenger, you don't need max HP capacity. You can always maximize a prop for running a less HP OB to deliver it's full max wot range and perform satisfactorily.

Happy Boating

 

[Watermann]

I agree with shrew. How can anyone blindly make a hp recommendation when no-one knows what type of boat the OP has?

I guess I'm reading his post and getting something different.

I think the OP is saying he has a big Triton and is wanting to down size to an aluminum boat that he doesn't have yet and is shopping around for.

My advice and opinion for the OP's aluminum boat he's looking to buy is to not go low on power and I gave him the ratings on my boats that I own as an example... not for a motor on a boat that he doesn't have.

Guess that's totally irresponsible.

 

[WIMUSKY]

I guess I'm reading his post and getting something different.

I think the OP is saying he has a big Triton and is wanting to down size to an aluminum boat that he doesn't have yet and is shopping around for.

My advice and opinion for the OP's aluminum boat he's looking to buy is to not go low on power and I gave him the ratings on my boats that I own as an example... not for a motor on a boat that he doesn't have.

Guess that's totally irresponsible.

Yep. It looks like we took his comments differently, at least between you and me. After reading it again, he probably doesn't have a different boat yet. I thought he did.

It would be nice if the OP would chime in after all the advice that was given, but he hasn't been active since he posted the ?.

And in other news............

 

[fhhuber]

Sorry, but recommending that people exceed the manufacturer's HP rating is irresponsible at best.

For one thing, the game wardens in many states will write a ticket for a large fine if you are grossly overpowered.

For another, any accident with the overpowered boat will lead to the overpowered boat owner being in for zero insurance coverage and lots of liability. The owner can also be charged with criminal negligence and any injury/death could be charged as a felony.

Nope... don't do it.

**********************

Result of being in the recommended power range and NOT at the max limit? You may not have as high a top speed, especially at full load. But speed capability is proportional to HP CUBED.

With all else equal, to double speed you need 8X power.

Appx 30% less than max rated HP is going to just be a few MPH difference from max rated HP.

But the lower power engine is usually lighter... allowing more useful load or higher performance at the same HP due to the reduced weight resulting in less drag. YOU MIGHT ACTUALLY BE FASTER with the lighter engine.

Prop selection can make a larger difference than the added HP...

 

[ondarvr]

Sorry, but recommending that people exceed the manufacturer's HP rating is irresponsible at best.

For one thing, the game wardens in many states will write a ticket for a large fine if you are grossly overpowered.

For another, any accident with the overpowered boat will lead to the overpowered boat owner being in for zero insurance coverage and lots of liability. The owner can also be charged with criminal negligence and any injury/death could be charged as a felony.

Nope... don't do it.

**********************

Result of being in the recommended power range and NOT at the max limit? You may not have as high a top speed, especially at full load. But speed capability is proportional to HP CUBED.

With all else equal, to double speed you need 8X power.

Appx 30% less than max rated HP is going to just be a few MPH difference from max rated HP.

But the lower power engine is usually lighter... allowing more useful load or higher performance at the same HP due to the reduced weight resulting in less drag. YOU MIGHT ACTUALLY BE FASTER with the lighter engine.

Prop selection can make a larger difference than the added HP...

1. Insurance companies still cover you, it's been stated by agents on this site and others, and when the open question is asked "does anyone actually know of a situation where a person wasn't covered for this reason let us know?", there's never an example of it.

2. There is no substitute for HP, going to the lower end is rarely good for anything but lowering the cost of the upfront purchase. I'm not saying you always need to be maxed out, but don't stray far from it.

 

[jbcurt00]

Guys, no one is suggesting overpowering a boat or exceeding max hp rating. In fact its all theoretical at this point, there is no boat to overpower.

 

[JimS123]

Saying that 75% of HP rating is generally recommended is merely one (or several) person's opinion. There is no boating organization that makes that an official policy or suggestion.

I have owned a boat that was powered at 40% of the rating and found it quite satisfactory for a family of 4 (two adults and 2 pre-teens.) That same boat at max capacity was downright dangerous to operate and that new motor was taken off in a hurry. I currently own 2 boats running max capacity motors and both run fine.

Bottom line is that all hulls are different.

 

[Sea Rider]

Saying that 75% of HP rating is generally recommended is merely one (or several) person's opinion. There is no boating organization that makes that an official policy or suggestion.

Jim, in fully agreement on this one along re propping under powered OB's for top combo performance under X given load. The issue is that boaters have made it another "Rule of Thumb" along the classic one that rules that there's a 200 rpm gain per one less pitch, which aint' so. Two same size boats with different hulls and weights will be max HP rated differently.

Seems I'm breaking the world record of under powered boating efficiency My combo is rated for 7 passengers and Max 50 HP engine, but with a maximized prop pitch is achieving full factory wot rpm with just 2 and middle wot rpm range with 3 adults with just a super tunned 18 HP. Achieve good hole shot, nice overall power and best fuel economy at 3/4 throttle range.

Happy Boating

 

[ondarvr]

People seem to not understand what "rule of thumb" means. It's a general guideline that is somewhat accurate most of the time, not an absolute fact that in every situation it will be 100% true.

 

[WIMUSKY]

^^^^^^ Very true. The OP still hasn't checked in to view all the responses. No use wasting key strokes if the OP isn't involved....