2 speed trans in boat

[Mastar]

Just curious why boat manufacturers don't put a couple gears in the boats 1low gear to take off and 1 high gear for cruising. I think I heard of MB boats or someone doing this, any one have any experience with them?

[jtryon]

MB had them in the early/mid-2000's but people report they were clunky when going into the higher gear

[Shaun R]

More weight, cost and complexity than it's probably worth. Cars need multiple gears because there is 100% traction between the tires and the road. With a boat the prop slips through the water and spinning it at a higher RPM does not always mean better performance.

A boat loaded down to surf can pull 4100rpm to do 10.4mph but dump the ballast and pull the tabs up and the same RPM is doing 35mph.

[EricB]

It all has to do with load, hp and running surface. Think of it this way; if it takes 3400 rpm (and 1/2 throttle) to move your boat 34 mph, shifting to a higher gear will load your engine down and thus will drop your speed. Your engine does not have the hp to maintain the speed given the load. It does not work the way a car does because a car is in a light throttle-cruise mode almost 80% of the time.

[ahhudgins]

Nothing would be better than being pulled up on a slalom ski, or getting ready to stand up from a barefoot start, and then have the transmission change gears.

[Stevext23]

Well, the new geared turbofans for jet engines means it is way more fuel efficient. Complaints previously of Clunkyness is just bad engineering. The PDK transmission from Porsche proves complicated trannies can be silk - and better.

[Mastar]

It all has to do with load, hp and running surface. Think of it this way; if it takes 3400 rpm (and 1/2 throttle) to move your boat 34 mph, shifting to a higher gear will load your engine down and thus will drop your speed. Your engine does not have the hp to maintain the speed given the load. It does not work the way a car does because a car is in a light throttle-cruise mode almost 80% of the time.

I get that there is way more friction to deal with than a car. But it seems like at a certain point cruising you have plenty of power you just need the prop to spin faster . At 4000 if you shifted to another gear and still had 3000 rpm you don't think there would be enough power?

[EricB]

I get that there is way more friction to deal with than a car. But it seems like at a certain point cruising you have plenty of power you just need the prop to spin faster . At 4000 if you shifted to another gear and still had 3000 rpm you don't think there would be enough power?

Good question. If your throttle position does not change though, you would still be using the same, if not more fuel. Depends on reading engine load (vacuum). Now if you could back off on the throttle like a car, then you'd be on to something!

[mikeg205]

lots of discussion on big boat forums - to get it right a variable pitch prop would be needed..

[Z71_Silvy]

I would imagine that at some point the hull can only go so fast and the prop can only be spun so fast (all things being the same other than a 2nd gear).

I would imagine the prop would eventually cavitate too from spinning too fast. I would think it can only move so much water.

[toddvdh]

It's not about spinning the prop faster. It's about keeping prop rpm the same while lowering the engine rpms.

Set cruise at 30mph in your car, then downshift 1 gear while not touching the cruise control. The car will stay the same speed but the engine rpms will increase. Now up shift 1 gear and the rpms drop again.

It would the same thing in a boat. It's doable I think. Getting the ratios right and shifting smooth would be the hard part. Think about it, 50 years ago 4 gears were plenty in a car. Now 6 isn't even enough.

[Mastar]

I'm always concerned about running high rpm while cruising, so I will usually cruise around at 25 mph getting passed by pontoons ! Is it much harder on engines to run at higher RPMs?

[lashburn1]

I'm always concerned about running high rpm while cruising, so I will usually cruise around at 25 mph getting passed by pontoons ! Is it much harder on engines to run at higher RPMs?

we have Wake Boats.....everyone passes us

With our 7.4 522 HP X46.. I can keep up with most boats heading up and down the River... but its like 4000-5000rpm... and I will pay for it at the pump.
my bigger fear is running out of Fuel before I get back home....

[MattsCraft]

lots of discussion on big boat forums - to get it right a variable pitch prop would be needed..

This does work - I had one one my Cobalt - Speed friction clutch built into the prop - It was like having a 2 speed transmission - Better hole shot & lower RPM for cruising. The only issue I had was if you went into a really hard turn with the prop in the higher pitch position the boat would bog down a bit until it would switch into the lower pitch position. You actually had to pull of the throttle so the clutch would switch, then get back into it. Expensive, but a really cool option for an I/O. I think I paid like $1000 for a 3 blade stainless at the time, looks like they double that now.

Always thought someone should make one for an inboard

http://aerostarmarine.aerostaraircraft.com/ppp.html

[LDA6339]

What about a big twin engine yacht with 4 props and 4 screws but only 2 engines and once the thing got on plane it switched from the two props for torque to the two props pitched for cruising? I wonder if that could work or be worth the time and money


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