Jet boat horse power vs out boarder horse power

[Bonmotwang]

I remember one of my friend told me that he had a small boat with 25hp out board engine, which was able to pull a skier.

The sportster is 130hp. Is there a comparison between out boarder power and jet power?

According to the numbers, my jet should be much more powerful his out boarder. But seems to have similar towing capabilities?



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[LouDoo]

I know I'm going to get a lot of flak about this but sometimes the truth hurts. Even though a jet boat has a lot of advantages over other boats efficiency is not one of them. I read a recent article in Boating Magazine, May 2014, which compared the three engine choices in the Scarab 195, here's how they finished, Mercury outboard first, Mercruiser stern drive second, and the Rotax Jet powered third.

I love my skis but for my boat I'm sticking with my old stern drive (Mercruiser) Bayliner.

Lou

 

[Bonmotwang]

I know I'm going to get a lot of flak about this but sometimes the truth hurts. Even though a jet boat has a lot of advantages over other boats efficiency is not one of them. I read a recent article in Boating Magazine, May 2014, which compared the three engine choices in the Scarab 195, here's how they finished, Mercury outboard first, Mercruiser stern drive second, and the Rotax Jet powered third.

I love my skis but for my boat I'm sticking with my old stern drive (Mercruiser) Bayliner.

Lou

Tell us they were close.
Just humour us lol


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[jhjesse]

I know I'm going to get a lot of flak about this but sometimes the truth hurts. Even though a jet boat has a lot of advantages over other boats efficiency is not one of them. I read a recent article in Boating Magazine, May 2014, which compared the three engine choices in the Scarab 195, here's how they finished, Mercury outboard first, Mercruiser stern drive second, and the Rotax Jet powered third.

I love my skis but for my boat I'm sticking with my old stern drive (Mercruiser) Bayliner.

Lou

Well what the heck does boating magazine know about boats?? :bs::lol: Just kidding!!

 

[Coastiejoe]

Outdrives and Inboards use props for a reason. They work... You can easily increase or decrease pitch and diameter based on needs.

They don't cavitate for the same type of reasons the jet drives do. Over-all they are much less likely to fail and in most cases are cheaper to buy and repair.

Props do what they do well. Been around for as for hundreds of years and have no real change to them with the exception of technology making them better.

 

[FinnExplorer]

I just read very old posting on the forum couple days ago where someone in Australia swapped 717 in Explorer for 55hp Mariner outboard and got pretty much the same performance. Bit depressing ain't it

 

[USAF_Pride]

I had heard at one point it has ~40% equivalency. I can't find any data to back that up tho.

 

[jhjesse]

Just spent the day on my challenger 1800 and honestly I don't care if a prop is better, my challenger is an absolute blast. Fun, fast, turns on a dime, jumps waves and I can throw my grandkids off the tube. :thumbsup:

 

[LouDoo]

Nobody said it was better just more efficient. I also spent the day at the lake and took a big chunk out of my aluminum prop., I've got a stainless steel prop. coming tomorrow.

Lou

 

[Coastiejoe]

Nobody said it was better just more efficient. I also spent the day at the lake and took a big chunk out of my aluminum prop., I've got a stainless steel prop. coming tomorrow.

Lou

You will like you SS prop.. You will find it more responsive and it will last for many years. We installed "slip hubs" on our go fast boats. They work pretty well. I was always worried about the fine line from slipping vs not slipping under normal heavy load. The slip hubs probably saved us 100 or better SS props.

 

[LouDoo]

Yep, I think the alum. prop. was a rebuilt, and I think where the rebuilder welded an new piece it just snapped off, about 1/4 of one blade just snapped off, straight across. You know in all the years I've owned ski's I've never replaced an impeller, I've filed a few nicks out but never replaced one. So there you go another advantage of a jet boat.

Lou

 

[jhjesse]

Dang Lou that's not good. Hopefully the SS prop will work out better. Someday I will have to go prop to make the Wife happy.
http://www.youtube.com/watch?v=LW69k7Y-QFM :thumbsup:

Good luck!

 

[brobertov8]

A neighbor friend told me his jet boat with two 215hp engines would do 65-70mph. Is that true? if yes, pretty fast for an speed boat jet, and it is jet powered.

But I got what you guys said. If was the same "little boa" with a single outboard 255hp, maybe would do the same or faster.

Personally I like v8 boats stern drive. I have on 454big block and I love it.... But for sure, outboard is more efficient.

 

[Dr Honda]

Back to the original poster.....


Your friend MAY HAVE pulled a skier with 25 hp... but I'm sure it wasn't fun, or easy. Also... did he say what kind of boat? if it was a 16' john boat (high flotation) and the skier was light, on 2 fat skis.... yes... I'm sure he gout out of the water. (and went 10 mph)


Honestly... it's a bad comparison.


As Lou said... there are some changes in efficacy's. But, I wouldn't go as far as to say a prop is MORE efficient.


Here's the deal.....

Prop:

1) There is more "Thrust" area. The physical diameter is larger for any given HP. That gives more physical "Pull".

2) Lower pitch. (in general) This also gives more pull at lower RPM's.

Note: The 2 above situations will give WAY more pull than any jet drive will ever develop. Also... it will help develop good speed on hulls that kind of suck. (like an old tri-hull) But, the more pull you are looking for, will scrub top speed off. It's the same as putting your car in a lower gear.



Jet Drive:

1) Smaller thrust area. But, because of that... you can turn more RPM's.

2) Higher pitch. Built for high speed.

3) The impeller is in a housing that necks down, to increase water velocity. When running on plane... the nozzle is out of the water, so it's not "Pushing" on anything. (like the prop) This is a pure action/reaction situation.

Note: the design of a jet pump doesn't come into it's own until the RPM's are high, and the boat is on step. Also... thrust is developed via the square or the RPM. ie... you will only make 20% thrust at 50% RPM. It's not linier like with a prop. But a jet drive can always (potentially) develop more top speed.



OK... here's a great example... My Islandia.

I have the 240 hp sport jet. My big-ass boat can hit 50 mph with only me on it, and minimal gear. Huricame builds a similar boat that uses an outboard. I've seen them with the 250 Opti. (10 more HP, and 500cc's larger) and they only hit 38~40 mph. Without a doubt, the outboard will jump out of the water better, and tow a couple skiers without an issue. AND get better MPG's at mid throttle. But, I can go faster over all, and I don't have to worry about logs that are under the water. (since I draft only 12" and the hurricane drafts +26")



OK... I know that's not an exact answer... but it's all what you want to do with the boat. You never see competition ski boats with a jet drive. Heck... you don't see them with an outboard, or I/O either. But, they use a prop that is WELL under the water. (for torque) But then again... you never see a comp ski boat trying to run in the shallows, or make a high speed pass.


I like jet drives... but no... they aren't the best at towing, because by the time they develop real pressure... you are going faster than most skier's want to go.


OK... with ALL that said... I had a highly massaged Sportster (single 95 hp 720 engine) and I had no issues pulling a wake board, or tube.



I know... too much info. But it's just the facts.

 

[Bonmotwang]

Hi Tony

This is a great write up. Much more valuable than a pure "comparison".
I love this type of "talking" articles. You just learn more and know why.