Prop torque.....

Tim M · 15 June 2006, 21:16

OK i recently fitted a 115 Merc to the back of my Osprey. When I get above about 4000 rpm the boat begins to "lean" to the left and feels quite flightly. Trimming out seems to cure the lean to a certain degree, although it still feels a bit uneasy, plus it causes the prop to cavitate.

What is the solution? Im currently running a 14" ali prop. Could changing the prop (maybe to a stainless one??) affect it at all. Failing that would taking the engine down a bit on the transom be of benifit. I am reluctant to do this for testing purposes as the engine is on the top holes at the moment, so the only way to move it any more is to drill new holes in the transom. I personally think this is going to be the way to go, as the cavitation plate is currently about an inch and a half above the bottom of the hull.....

What do people think?

LURCHER · 15 June 2006, 21:25

Tim
I had the same problem when I first got my Humber, the problem was caused by the engine being fitted slightly wrong . It was not vertical on the transom and at speed the engine in effect tilt itself to vertical which meant the baot listed . I have now had a alloy plate made to cap the transom and the engine refitted using a jig to drill the holes . Result is perfect with the added bonus of a complete new feel to the boat and no ventilation in turns , this may be the stainless prop but she now rides true .this post refers http://www.rib.net/forum/showthread.php?t=14063

Jimbo · 15 June 2006, 21:27

I would have thought on your rib with that engine a slightly higher pitch might be appropriate.

Ref: The lean, sounds like the anode trim tab behind the prop might be out of place, or the engine may not be mounted dead vertical on the transom.

Is the anode in good nick, or is it a bit degraded?

Tim M · 15 June 2006, 21:30

Quote:

Is the anode in good nick, or is it a bit degraded?

This model has a plastic trim tab not a zinc one. I have to say this was something that I had overlooked until now. I'll try adjusting it when I take the boat out on Monday.

Jimbo what size prop are you using. I know your boat is a bit heavier but it is a similar setup.....

Lurcher - I'll get a tape measure and check that the engine is on dead square tomorrow.

Richard B · 15 June 2006, 21:51

Check the offset as well... is the engine central or slightly to one side? I would guess that it may need to be 30-50mm to the right (starboard), but someone who's set up a similar sized engine on that type of hull would be able to advise you more accurately.

ADS · 15 June 2006, 22:20

Thats alot of power on a small boat Tim, I would think the 14" is a little under proppedd, im running a 17" on the Tohatsu.
Likely to be the set up of the engine I would think, may be worth offsetting with all that power, a mate had a similar problem with a 75HP Mariner on a Valiant 5.2m, the boat became known as Eilean!!

Richard B · 15 June 2006, 22:21

Quote:

Originally Posted by ADS

the boat became known as Eilean!!

Surname Dover?

ADS · 15 June 2006, 22:24

Quote:

Originally Posted by Richard B

Surname Dover?

Tim M · 15 June 2006, 22:32

I don't know if the engine is off-set. If it is it's only by a small amount (it looks central). I just bolted the new engine onto the exsisting holes....

I don't know anything about propping boats. What sort of pitch would be more suitable? I suppose it would help if I said what the engine tops out at rev wise, but I can't confirm this until Monday.

Downhilldai · 15 June 2006, 22:33

What engine was fitted before? How did it perform with the previous motor?
Did you use existing mounting holes?
Does it lean to port while accellerating onto the plane, or only at high RPM?
Presumably, the boat is loaded evenly?
It does sound like insufficient offset, from what you have posted.

Alice · 15 June 2006, 22:36

Quote:

Originally Posted by Downhilldai

What engine was fitted before? How did it perform with the previous motor?
Did you use existing mounting holes?
Does it lean to port while accellerating onto the plane, or only at high RPM?
Presumably, the boat is loaded evenly?
It does sound like insufficient offset, from what you have posted.

' Coz oddly, prop torque has more of a longitudinal effect on Downhilldai's boat.

Tim M · 15 June 2006, 22:36

It had a Mariner 75 fitted before. I have no idea how it performed becasue I removed that engine as soon as I bought the boat.
It only leans over 4000 rpm - its fine below that. Yes the boat is evenly loaded....

Im now worried that I've over-powered the boat. 115 is a lot for such a light narrow boat, although it is rated for it....

ADS · 15 June 2006, 22:42

Quote:

Originally Posted by Tim M

It had a Mariner 75 fitted before. I have no idea how it performed becasue I removed that engine as soon as I bought the boat.
It only leans over 4000 rmp - its fine below that. Yes the boat is evenly loaded....

Im now worried that I've over-powered the boat. 115 is a lot for such a light narrow boat, although it is rated for it....

QUOTE=Tim M] I don't know if the engine is off-set. If it is it's only by a small amount (it looks central). I just bolted the new engine onto the exsisting holes....

I don't know anything about propping boats. What sort of pitch would be more suitable? I suppose it would help if I said what the engine tops out at rev wise, but I can't confirm this until Monday. [/QUOTE]

Probably is slightly over powered but that doesn't mean you have to use all of it. I would think that it may be a little overpowered and you may wish to considern offset to counteract the torque from the prop, that was why this Valiant healed over at every chance over waves, the dealer moved the engine in the end (new boat). But before you start drilling holes and turning the transom into a piece of cheese I would contact Osprey to see what they reckon, I believe David Manning (posts on here) has rigged a number of Ospreys may be worth pm'ing him.

Downhilldai · 15 June 2006, 23:05

Agree with this - a comparable rigged boat which is well-balanced would be a good starting point to work from. David Manning is a bit of an Osprey guru - he may well be able to give some advice on this, or you could try speaking to outboard dealers who have rigged similar boats? Richard B's suggestion of 30-50mm offset sounds about right to me. Moving from a 75 to a 115 is quite a step on a boat of that size, so a degree of additional offset would probably be required.

Hightower · 15 June 2006, 23:28

Tim, you mention 14" for the prop, is that Diameter or pitch? I'd have thought diameter. For a 115 on a 5.6 a 17" or 19" would be suitable, they are very common sizes and I'd have thought your engine would have one of these. Where'd you buy it from?

You have to also remember that you've never run this RIB before, so the lean could have been there before with the Mariner....Just a thought!

Check all obvious things first. Outboard Straight. Cav plate level to bottom of hull. Is Outboard in centre or offset, how big an offset and to which side. Water in hull on side that is lowest when tilting occurs. Heavy Items on deck that are offset.

Just a few thoughts.....Hope you get your prob solved.

Tim M · 15 June 2006, 23:38

Andy, as I said I'm a prop novice! There is a sticker just above the prop with "14" on it which refers to the size of the prop. This is either diameter or pitch, I assumed it was the later.

The boat is definatly balanced and running level. I am 100% sure the problem is due to the engine in someway. I'm going to spend a bit of time tomrorow measuring everything to see exactly what is going on. I'll check the numbers on the prop too....

Tim M · 15 June 2006, 23:45

Quote:

Cav plate level to bottom of hull

Should I expect to have to trim the engine out in order to get good, stable performance (to counter-act the prop torque)? If the answer is yes then I think I need to move my engine down at least an inch so that the plate is nearer the bottom. As I said at the moment it is quite a bit above due to the lack of any more holes to go down with. This would allow me to run it at higher revs trimmed out without cavitating....

Hightower · 16 June 2006, 00:44

Trimmed out a little with outboard right angles to ground the cav plate should be level with bottom of hull, this is a basic set-up starting point. Also Offset (seek advice from Engine/Boat Manufacturers about this). Mine is in the centre of the transom and works a treat 90hp on a 5.3m.

I would start from there otherwise you could go around in circles. Incidently the trim/anode at the trailing edge of the cav plate is to counter torque steer but won't counter it in all trim angles/speeds, you'll have to set this up to a neutral position to suit yourself. This shouldn't really affect the boat listing to one side at 4000+ revs though.

BogMonster · 16 June 2006, 00:59

Quote:

Originally Posted by Tim M

Andy, as I said I'm a prop novice! There is a sticker just above the prop with "14" on it which refers to the size of the prop. This is either diameter or pitch, I assumed it was the later.

The boat is definatly balanced and running level. I am 100% sure the problem is due to the engine in someway. I'm going to spend a bit of time tomrorow measuring everything to see exactly what is going on. I'll check the numbers on the prop too....

If it is like mine it will have the pitch and dia stamped in to the metal of the prop. Mine had 14x17 which is 14" dia and 17" pitch. I think mine can do with a 19 and could probably handle a 21" as it seems to have bags of power and hits max revs quite quickly, but a 19" is on order at the moment to see how that goes, I don't know much about it either but am feeling my way around

Hightower · 16 June 2006, 01:12

Have a look at this Pic. Don't know what set of holes you used (got a few to choose from) but they do seem off centre. Could be a trick of the Photo, but what do you think?