Thread: transmissions

Ok, I tried doing a search and didn’t find an answer I was looking for.
My newbie question is: why don’t you see transmissions on boats?

I fully understand that there is not a need for extra torque when starting, and that a prop works differently than the wheels of a car when it comes to revolutions and "slippage".
I am also aware that there would be little if any extra speed gained by adding a transmission because faster prop revolutions would only cause cavitation.
I am mostly interested in the fuel economy of an engine fitted with a transmission. It seems if you could get a 2-speed overdrive-type transmission, once the boat is planed out and at cruising speed you could "shift" it into overdrive thus lowering engine rpm and increasing fuel economy. This seems like it would greatly increase your range.
I don’t think this would have a very practical application for recreational and sport boats; I am more thinking along the lines of yachts and cruisers.
I have heard arguments about added complications and higher maintenance costs. However, it seems that a relatively simple (reverse -> neutral -> forward -> overdrive) transmission would not be unfeasible. There would also be very little stress put on the transmission because the prop can “slip” through the water with very little resistance.


These were just a few thoughts I had today while thinking about boating and cruising

opinions?

Where are you getting the information that there aren't transmissions on larger boats? They exist in both small and large. In fact, I just paid a $22,000 bill (no, that's not a typo) to have one (1) of ours rebuilt.

They're typically referred to as gearboxes on larger boats (yachts).

I'm really confused on where you're getting your info, and or basing your theories off of for overdrives, lower RPM's, and longer ranges.

You can't take a larger diesel engine, pipe it through a gear to continue to lower the RPM which in effect would (in theory) extend your range. Remember, there is a power band of an engine, and the boat needs to stay / remain in that. Then you have the whole hydrodynamics thing with pushing a hull through water. This opens a rather large discussion in and of itself.

haha im not exactly getting my info from anywhere.
i do not have a whole lot of experience with larger yachts, and i havent heard much of anything about marine transmissions (hence the "newbie" statement). i was thinking that lowering the engine RPMs while still having the prop spin at the same speed would improve range. i wasnt thinking down to idle speed of anything, just lower...
obviously, there are many who know a TON more about this kinda thing (you included) and i just thought it would be an interesting discussion...
even if i am totally wrong i would still like an explanation as to why, because i cant wrap my head around why it wouldnt work for a yacht once at cruising speed lol.
thanks for the reply

Typically what you'll do is gear up your prop speed in relationship to the RPM of the motor. I'd have to look exactly, but in our smaller boat (65') it's something like 1.5:1. But that's keeping RPM at 1200 or so (being a slow cruise at 6 kts) where at WOT or full speed, we're running at 2400 RPM and 22 kts. I'd have to check numbers again, but that gives you the general idea.

I think the general concept is that the prop is only so efficient, no matter how fast you spin it, before it a) cavitation begins or b) throws a blade/ distorts, and c) (being the most common) no matter how much power you put to the hull of the boat, it's only going to go so fast, be it a planing, displacement, or semi-displacement hull, before there is a diminishing point of return. This opens the conversation of thermodynamics and hull efficiency, which is a subject I know little about other than at a very high theoretical level.

I hear what you're saying about running at a lower RPM, thereby over-driving your prop to gain range, but again, even at the lower end of the spectrum, there is a diminishing point of return in the power and torque band in which the engine needs to run.

Sot / crap / carbon builds up in the engine at low RPM's, thus every few hours at lower speed cruising, we turn everything up for a few minutes to 'clean it out' so to speak.

i totally understand what your saying. haha i know so little about motor yachts, i always thought the drive ratio was 1:1. i was thinking that you could have an RPM limiter on the prop shaft itself, for example if the final drive ratio for a prop was say 2:1 an engine would be turning at 1200 turning the prop at 2400 rpm. wouldnt the boat move the same speed as a 1.5:1 ratio drive with an engine running at 1600 rpm? if the prop was not allowed to exceed the cavitation speed, wouldnt it not matter how quickly the engine was turning (other than being well under the powerband of certain engines).
haha i dont mean to be a pest or extra thick headed or anything, it was just something i couldnt figure out lol.

2- speed transmissions are just now working their way into modern HP boats.. Most transmissions simply allow you to select a gear while the motor is still idling. Others, dont. The ones that dont allow that are called "crashboxes". The motor has to be shut off to select the gear. These are typically found on high HIGH horsepower configurations that have been ultimately too hard on trannies.. My good friends Jim Lee (Freedom US-1 46' Skater) and his engine builder are making a set of trannies now that are 2-speed.

The two speeds allow you to keep your engine RPM up a little higher during docking manuevering, launching and planing. Once the boat is on plane, you hit the switch and it grabs second gear. But as far as allowing the engine to shift thru 3 or 4 gears once on plane (which is what I believe you are asking), has been impossible because the lower you get in the RPM, the less torque you make. It takes a LOT of torque to turn big wheels and to lift a boat high out of the water to reduce the drage. Once you start lowering that torque curve, you have to switch to smaller props and you lose a lot of the properties you were shooting for..
Also, if you were to come out of the water and back off the throttles right before re-entry, you'd have to slow down and plane out all over again.. Which would kill the entire point of the run..

Hope that helps.

thats exactly what i was wondering about. a 2 speed tranny that would allow the engine to turn a bit slower once planed. i wasnt thinking down to idle, just backing it down a bit. being more of a gar guy i was always amazed at how hard people seem to run their boats. my idea was centered around something that could shift fast enough so as not to ruin the plane once at cruising speed

thats a novel idea, but show me a transmission that can shift at 7k rpm with 1500hp and 1200ftlbs tq in front of it..

haha you have a very valid point there

MB Boats is putting a 2 speed transmission in their whole lineup if I remember correctly. I've never used one, but my understanding is that it is an "either/or" system where you choose either low speed/pulling or high speed/cruising when you first put it in gear. Keep in mind that these are boats in the 350-400 HP range and they're vDrives so you don't have to worry about stuffing all the extra equipment in an outdrive unit.

It seems logical to have one ratio that gives you the best cruising efficiency and one that gets it to top speed without over-revving the engine, but I'm sure it is going to be pricey on larger boats with more power. Especially with the shift-on-the fly systems like the one Waterboy is talking about.