Estimating top speed with a horsepower increase, part 2

[waterinthefuel]

Rather than bury my question at the bottom of a thread about someone else's boat, I'll make my own.

I have a 13.5 foot fiberglass boat. It has a 28 on it and does 27.5mph wide open. This is GPS'ed. That's rollin' in a small boat. The top speed actually doesn't even go down with 2 people in it. That being said, it's rated for a 40 (I know, that's nuts) and I have been hard and looking at putting a 40 on it just to have the fastest 13 foot boat on the lake. What kind of top speed would you guess I'd get? And what about fuel burn? Is it worth the money? The 28 I've had since new in 1993 and it runs perfectly and always has with absolutely zero problems.

My dad says 27mph is plenty fast enough in a little boat and I'd be nuts to get rid of such a reliable powerplant as my 28 to get a 40 that's questionable (I can't afford a new 40, a used would be my only choice). The boat's freeboard in the back with my dad, not a large person by any means and the 28 is about 3-4 inches. I'm worried the weight increase might bite me bad.

What do you think?

 

[Mischief Managed]

Re: Estimating top speed with a horsepower increase, part 2

Assuming the same prop efficiency and load, it should go 32.9 MPH with 12 more HP.

 

[Bondo]

Re: Estimating top speed with a horsepower increase, part 2

My dad says 27mph is plenty fast enough in a little boat and I'd be nuts to get rid of such a reliable powerplant as my 28 to get a 40 that's questionable (I can't afford a new 40, a used would be my only choice).

Ayuh,... Listen to your Dad,...

Reliability isn't worth a couple of mph...

 

[Philster]

Re: Estimating top speed with a horsepower increase, part 2

If speed = happiness life would be so simple.

Yes, coming from me, a guy with a boat built for speed, so easy for me to say -- right? But I have a pretty stock, dead-on basic and reliable engine. The headers are just an upgrade that improves reliability. Risers were overdue anyway.

But I go 63-64 mph, all day and night, while a whole bunch of yahoos are supercharging, and swapping out cams, intake and ECUs to get near 70. Meanwhile they get about 75% less boating time.

The lesson here? Nothing beats reliability/safety when it comes to boating. Reliable and dependable 28mph is better than unknown reliability with 32 mph, and reliable and dependable 63mph is better than unknown reliability with 68mph.

 

[5150abf]

Re: Estimating top speed with a horsepower increase, part 2

Where did we learn you boat is rated for 40 horse?

That just sounds like way to much power for that small of a boat.

You are more than likely going to take a nice well handling boat and turn it into a bad handling boat that is really fast in my opinion.

 

[stinkbait85]

Re: Estimating top speed with a horsepower increase, part 2

Here's a comparison. I have a 16ft aluminum bass boat. When I bought it ithad a 55hp suzuki 2stroke on it and it would bounce between 35 and 40 gps'ed. I have since put an 85 merc on it and it now bounces between 48 and 52,53 somewhere in there depending on water conditions. Now these figures are with just myself in the boat, nothing else.

 

[waterinthefuel]

Re: Estimating top speed with a horsepower increase, part 2

It's written on the data tag that it's rated for a 40. If you look at the boat you would never guess a 40, maybe a 25. Maybe. It's a 13.5 foot boat but the first 3 feet is unusable pointed "deck" where I have my trolling motor mounted. So the usable part of the boat is about 9 feet long. Within that 9 feet are 2 pedestal seats and a livewell between them. When it's on plane about 4 feet is in the water. And that's with 2 people and a 28.

I think it's got enough power right now. It's only rated for 2 people anyway. And it planes right over with 3 adults in it.

And yes, it does handle well. It USED to have a Yamaha 40 on it when it was new (and before we got it) and we found it with no motor and the floor cracked. I think thats just too much motor for that little boat. We repaired the floor and transom and stuck a new 28 on it and it seems like a perfect match size wise. Its a 93 model, bought brand new in '94 and has been untouched except for a carb repair (factory didn't have it right, after my dad worked on it it ran about 5mph faster than it did before) and the usual water pump, spark plugs and lower unit oil. It doesn't look like it would be a bullet but I can keep up with most 40's on the lake and some 60's on larger boats. I can outrun most any party barge with anything under a 115! LOL

 

[trendsetter240]

Re: Estimating top speed with a horsepower increase, part 2

It's written on the data tag that it's rated for a 40. If you look at the boat you would never guess a 40, maybe a 25. Maybe. It's a 13.5 foot boat but the first 3 feet is unusable pointed "deck" where I have my trolling motor mounted. So the usable part of the boat is about 9 feet long. Within that 9 feet are 2 pedestal seats and a livewell between them. When it's on plane about 4 feet is in the water. And that's with 2 people and a 28.

I think it's got enough power right now. It's only rated for 2 people anyway. And it planes right over with 3 adults in it.

And yes, it does handle well. It USED to have a Yamaha 40 on it when it was new (and before we got it) and we found it with no motor and the floor cracked. I think thats just too much motor for that little boat. We repaired the floor and transom and stuck a new 28 on it and it seems like a perfect match size wise. Its a 93 model, bought brand new in '94 and has been untouched except for a carb repair (factory didn't have it right, after my dad worked on it it ran about 5mph faster than it did before) and the usual water pump, spark plugs and lower unit oil. It doesn't look like it would be a bullet but I can keep up with most 40's on the lake and some 60's on larger boats. I can outrun most any party barge with anything under a 115! LOL

I had a 13.5 ft runabout with a 1991 40hp yamaha. The plate rating on my boat was also max of 40hp. It was a perfect match. At first I had a 25hp evinrude on it and it was fine with just me. When I wanted to take 2 passengers and extra fuel the 25 would have a hard time. Additionaly, the extra HP will allow you to cruise a lower RPM thus saving on fuel.

If you are operating the boat in calm waters then by all means, put on the max rated engine for the boat. Stick with a 2-stroke as it will be significantly lighter than a 4-stroke in that HP range.

Cheers!