V-Drives

[DNT99611]

Can the V-drive be set off vertical? If it could, and a down angle tranny were used, could you then get an almost flat prop shaft and still have the engine in the stern?

 

[diver dave]

Wow, pretty small bearing on the output shaft, opposite the flange[emoji848]

 

[Slowmo]

I'm sure the other engineers on the forum have let you know that ALL gears have a power loss. The amount of loss is a function of the gear design and speed. Since the bevel gear for the V-drive can also function as a reduction gear, there's really little efficiency difference. Direction of power flow isn't a factor, just the number of gears and their design. Those who think that a 90 degree direction change has a big loss may be confusing the change in direction with the type of gear. Rear drive automobiles use hypoid gears, these are less efficient due to a sliding-rolling contact.

 

[Nomad Willy]

kapnd wrote;
“Installing vee drives is a decision driven by space and weight distribution factors.”

Re space no new space is created. But space is made available amidships where it usually is considered more valuable.

“Weight distribution factors” is a much more serious matter.
Ideally you should be able to pull your engine and trans out of the boat and the new waterline would be parallel to the W/L w the engine installed. Most all boats will vary a little but having the weight bias a small bit aft is better. Perhaps almost essential. But reinstalling the engines far aft (as in most all Vdrives) is bad to very bad IMO.

Ideally and desirably the CG would be very close to the center of buoyancy. The downside to having the weight aft is mostly related to the boat running at inefficient angles of attack or running angles. Such boats will frequently be reluctant to get on plane. Being a trawler forum we’re not concerned about that but running at high angles of attack will be quite to very inefficient re fuel burn.

And I might add that most Vdrive boats will have tanks of fluid, batteries and other heavy equipment well fwd to compensate for the aft CG resulting from the Vdrive engines. One would think (as I did for years) that having all the heavy stuff in a boat in the middle of the boat would be ideal. Not so. As the sea heaves and rolls a boat is set in motion or should I say motions. A boat w all the weight amidships and little at the ends will be way too prone to yawing and pitching. EXCESSIVE yawing and pitching. Also a boat w most of the weight at the ends is a pig at the helm. Will be very reluctant to turn and once the yaw for a turn is in motion will be difficult to stop or reverse the action. And in pitching a lot of weight in the bow will send the bow deep into a steep sea.

Re the above it’s obvious that a seasoned NA needs to incorporate into a boat a good balance of weight distribution to give a boat desirable motions at sea and minimizing undesirable motions.

Above I probably painted the vdrive boat as undesirable. They are for many other reasons not mentioned but some Vdrive boats are quite to very good designs. They come to pass as the result of unusual circumstances like having a very low power requirement (like a sailboat or possibly a FD trawler) whereas a small and light engine like many Yanmars could result in excellent weight distribution. But w big 6 or 8cyl engines it’s not likely. Despite liking outboard powered boats a lot l’m very prone to say engines belong amidships in any arrangement that is somewhat close to ideal. And ideal weight distribution something one shouldn’t stray very far from.

 

[Art]

kapnd wrote;
“Installing vee drives is a decision driven by space and weight distribution factors.”

Re space no new space is created. But space is made available amidships where it usually is considered more valuable.

“Weight distribution factors” is a much more serious matter.
Ideally you should be able to pull your engine and trans out of the boat and the new waterline would be parallel to the W/L w the engine installed. Most all boats will vary a little but having the weight bias a small bit aft is better. Perhaps almost essential. But reinstalling the engines far aft (as in most all Vdrives) is bad to very bad IMO.

Ideally and desirably the CG would be very close to the center of buoyancy. The downside to having the weight aft is mostly related to the boat running at inefficient angles of attack or running angles. Such boats will frequently be reluctant to get on plane. Being a trawler forum we’re not concerned about that but running at high angles of attack will be quite to very inefficient re fuel burn.

And I might add that most Vdrive boats will have tanks of fluid, batteries and other heavy equipment well fwd to compensate for the aft CG resulting from the Vdrive engines. One would think (as I did for years) that having all the heavy stuff in a boat in the middle of the boat would be ideal. Not so. As the sea heaves and rolls a boat is set in motion or should I say motions. A boat w all the weight amidships and little at the ends will be way too prone to yawing and pitching. EXCESSIVE yawing and pitching. Also a boat w most of the weight at the ends is a pig at the helm. Will be very reluctant to turn and once the yaw for a turn is in motion will be difficult to stop or reverse the action. And in pitching a lot of weight in the bow will send the bow deep into a steep sea.

Re the above it’s obvious that a seasoned NA needs to incorporate into a boat a good balance of weight distribution to give a boat desirable motions at sea and minimizing undesirable motions.

Above I probably painted the vdrive boat as undesirable. They are for many other reasons not mentioned but some Vdrive boats are quite to very good designs. They come to pass as the result of unusual circumstances like having a very low power requirement (like a sailboat or possibly a FD trawler) whereas a small and light engine like many Yanmars could result in excellent weight distribution. But w big 6 or 8cyl engines it’s not likely. Despite liking outboard powered boats a lot l’m very prone to say engines belong amidships in any arrangement that is somewhat close to ideal. And ideal weight distribution something one shouldn’t stray very far from.

Eric - You lose me then you gain me... then you lose me then you gain me... your inputs are always interesting.

 

[rslifkin]

Keeping the heavy stuff out of the ends is good, but I can't say I'd agree that keeping the engines as close to the center of buoyancy as possible is ideal. I'd worry more about that for the variable loads like fuel and water, as that should make the boat's behavior more consistent as tank levels vary.

On my boat, for example, I have about 2300 lbs of engines and transmissions. With full tanks, I carry 2650 lbs of fuel. So switching the position of the fuel tanks and engines wouldn't impact the overall weight distribution of the boat much (although engine access would suck, but that's a different issue). But the variable weight would now be much closer to the CoB, meaning that weight distribution would change less with changing fuel load. Currently, burning off 1000 lbs of fuel makes a noticeable difference in the boat's resting deck angle (but a surprisingly small difference in performance). Of course, this is a lot more noticeable on a boat that's burning fuel at nearly 200 lbs / hour for planing cruise compared to a slow trawler burning 20 lbs / hour.

 

[diver dave]

Lbs per hour. There must be a flyboy in our midst. [emoji108]

 

[rslifkin]

Lbs per hour. There must be a flyboy in our midst. [emoji108]

Surprisingly, no. I just figured that if we were talking about weight distribution, knowing fuel in lbs was more useful than gallons.

 

[Ski in NC]

For hull speed running, you want the weight up more forward. Most hull shapes at slow speed suck when a$$ heavy. So vee-drives are not the first choice for trawler type service. Unless designed that way from the git-go with proper loading in mind.

On planing boats, it is a different story. Planed out, you want weight more toward aft (but only up to a point). Too heavy aft and it becomes a pig when trying to get over the hump. You definitely don't want a planing boat bow heavy.

 

[Baker]

I will say I have a buddy that used to have a Sea Ray 47 Sedan Bridge. Great boat. But it had V drives and the engines being aft made the boat stern heavy. And the fuell efficiency suffered tremendously. At 20kts that boat would burn over 50GPH!!!....with "efficient" QSC 600s. I was sure there was something wrong with the boat. He showed me the book and it was right on the money. The older 480 Sedan Bridges with QSM11s burned significantly less than that(in the 30s gph) And I think they were even heavier boats.

 

[Art]

We had a 1973 31' Uniflite... with twin screw V-8's, V-drive, sedan, flybridge sport fisher pleasure boat.

Really good handling vessel. Would jump onto plane at the drop of a hat. WOT in the high 20's. Easy, affordable cruise at near 20 mph.

I've taken fully equipped film crew onto SF Bay for doing a trailer on a save the ocean movie.

 

[Nomad Willy]

Art,
The Uniflite hull had a warped bottom. The only trawler I can think of that does is the old MS34. “Warped” whereas the bottom looses deadrise as one goes aft. In the case on the MS it finally gets flat at the transom but fwd they have lots of deadrise. A high degree of warp means going from deep V-like fwd to little or no deadrise aft.

Warped bottoms are great for a shape that will promote a boat to run more level. And unless the wetted surface is increased too much it will promote efficiency also ... at a good planing speed. The warped bottom is kinda like a huge trim tab. And racing down the face of a big wave a warped bottom is frequently not a good thing as you could loose directional stability, burry the bow or even broach.

The Uniflite “jumps on a plane” because of their warped bottom and deeply submerged transom. The wide beam at the chine even gets into the act.

And per this discussion a warped hull would be excellent fora V drive.