Merc vs Rotax -- handling?

[henryb]

This is just a question... for anyone who has had the opportunity to drive both the Merc engined Seadoo boats and the prior dual Rotax Seadoo boats. I have heard comments that they are "completely different animals", regarding steering, stability and handling... I would be most interested to hear exactly what the differences are!

 

[Dr Honda]

Wellllllll...............


I wouldn't say it's like that.


The older 2-stroke boats were light, and could be thrown around like a big jet ski. And, the new 4-stroke boats are smooth, and relatively quiet. The Merc boats... well... are the Merc boats. (kind of in between)

OK... I've driven the 2000 with both the twin 800 Rotax, and the 240 EFI Merc. To me... they felt similar. The hole shot on the Merc was better. It has a larger pump, and was in good shape. The twin 140mm pumps move water, but are prone to cavitation with the heavier boats.

I've also driven all the "Small" boats. (Sportster with a single 720, Challenger with a single 800, a Sportster with twin 720's, the new Speedster with the N/A and supercharged engines) Hands down... the fastest was the supercharged speedster. BUT... the older Speedster just felt more fun. Probably because it's 500 Lbs lighter. I personally had a single Sportster, that was "Massaged" a little... and probably ran better than the day it rolled off the showroom floor. I loved that little boat. It was under 1000 Lbs !!! Yes... my 4 passenger boat weighed less than a modern 2 passenger PWC, and had room for a day's worth of crap. But, I guess that's why I bought my Polaris Genesis. It's also 4 passenger... makes more power... and has a bunch of storage.

I have not driven a modern Islandia with twin 4-stroke Rotax engines yet... but looking over the numbers... it wouldn't be a contest in the speed department. The 2010 version was known to hit 55 to 60 mph. Mine will do about 49 mph with just me on it, and the top folded back. BUT... then again... if you have 11 friends sitting on it... how fast are you really going to go??

On top of that... there have been hull changes. I know on the islandia, they changed the lower hull twice. So, there's the original... the Gen 2 that had less bow rise, and would ride flatter... and the Gen 3 with the Rotax engines. So... regardless of the power... they all ran a little different.

Other than those, and knowing there were hull changes... there really isn't many crossover boats to give a real comparison to.


I guess if I knew your ultimate goal of the question... I could help you better with it.

 

[henryb]

Thanks Tony, I didn't have any real goal for this question other then an interest in opening a discussion on this topic... As it was the mechanic at the marina that I go to that made that quote... and he didn't really substantiate what he meant.

Now that I've had the Challenger 1800 210HP for a summer, I know it's characteristics:
- decent hole shot, planes quickly approx 3 secs
- pulls to the left (fixed with a 2001 offset nozzle, hopefully!),
- porpoises quite a bit (fixed with extended ride plate),
- if people weight is not well balanced, it carves turns like a knife one way and turns like an outboard the other way!
- it can spin on a dime, if weight is well balanced,
- skitters left and right a bit when on plane at cruising speed, a bit more speed normally fixes that.

I find this boat really rides best at 3/4+ throttle (40+mph)... Flat, straight, carves or spins, very responsive... Outstanding!

 

[Dr Honda]

OK... I get it.


Unlike a car... a boat "Flys" in the water. And... because of that... every hull has it's sweet spot. (Balance, speed, weight) I don't care how well someone say's their boat works... it's all relevant until they get HUGE. (over 40') Even my big boat is at the mercy of the people on board. I find on really hot, sunny days... everyone wants to be under the top. And... since I have enough room for at least 7 people under the Bimini... I can have a lot of weight in the back. When that happens... it takes almost full throttle to get it on a step. (otherwise, it rides nose high) If a few people sit up front... then it comes out the water flat... and rides nice.

Now... with that said... my boat has ZERO tendency to porpoise. I don't care what speed I run, or how much weight is in it... it simply rides flat at speed. I imagine that the overall weight acts like a shock absorber.


Now... my old sportster would bounce a little... but not too bad. But it dis have a speed that it didn't like to run. Basically, there wouldn't be enough lift to get the boat flat... so it would sit a little to one side. (on a chine)

 

[henryb]

Thanks again Tony... you made some interesting points about the Islandia hull design has gone through some changes over the years. Do you know if the Challenger 1800 has gone through any hull changes and when? I'm guessing there is probably a difference between the 2 engine and single engine hulls?

 

[Dr Honda]

I don't think there was a reall change in the 1800 hull. Obviously, they would have the rear section changed just to mount the pumps... but the forward section stayed the same.

With that said... as you know... as small change to the rear can have a large change in how it handles.

Next time you have to boat on the trailer... post a picture of the transom. I may have a few I can post of the twin engine. (I had to send pics on a boat I worked on last summer)

 

[henryb]

I will be testing my new Hoist system that I mounted in the boathouse this weekend... I will take some pics and post them up.

 

[henryb]

Here is a photo of the back of the hull... Lifted out of the water.

 

[Dr Honda]

pretty close. But there is a small extended section in between the pumps. AND... you get more "Skeg" action with 2 pumps.