New prop causes veer to port - solution??

[sailingdeacon]

After years with an old 3 blade feathering prop, I replaced it with a new design folding prop. The new blade pushes the boat at higher speeds and has very little prop walk in reverse (right hand prop). The problem is that NOW under power the boat veers to port - significantly - and at the upper end of the speed range it is considerable. Max rpm under load is 2900-2950 of 3200 max - and the tac is correct. This 35' boat has no pull left or right if not under power. The shaft is centered and not angled either way. The rudder is perpendicular (to the eye at least) There is more than enough room between the prop and the hull.

It never did this with the old prop in any conditions - although at cruising rpms the speed was slower by about 1/4 to 1/2 knot.

After many emails with the manufacture, including photos, they are quite perplexed, but are suggesting a reduced pitch of 1 inch - which will get the rpms to max - at least.

Or would a reduction in the diameter of 15" be better??? Will the pitch reduction help?

[djodenda]

What kind of boat?

[Capnblu]

Steer to Starboard!

[sailingdog]

It would help if you said WHAT PROP YOU BOUGHT. Someone on the board may have experience with it... but if you don't say what brand, make and model it is, how can anyone help you???

It would probably also be wise to give the pitch/diameter of the prop as well as the type of boat and engine that you have.

I highly suggest you read the post in my signature... the more relevant information you can give generally results in better suggestions. As they say in the Computer Science field... GARBAGE IN ===>>> GARBAGE OUT.

[Rockter]

sailing...

If the pull is unacceptable to you, put the old prop back on again. Either that, or it's expensive trial and error. Finer pitch might help.

[Idiens]

I observe the same effect on my boat since I re-engined and reversed the Maxprop rotation direction in the process. But on my boat, it shows itself more in terms of pressure on the rudder/wheel at high rpm. Since I adjusted the prop to give 3000 rpm at max throttle setting, but reach hull speed at about 2600 rpm and normally cruise at 2000 rpm, the effect does not worry me.

[beej67]

Originally Posted by sailingdog

GARBAGE IN ===>>> GARBAGE OUT.

Yeesh, Dog, a simple "Could you be more specific?" would have probably sufficed.

[sailingdeacon]

I doubt naming the prop will help since there was only one brief response to my query here for any experience with the prop sometime ago (Search on Slipstream prop) . I did not describe the specifics since the theory of what causes a prop/boat to act like this is generic in my opinion. However here are the details:

It is a relatively new design, "Slipstream" folding 2 blade prop made in Australia. the boat is a 1987 Ericson 34 shoal fin keel, exposed rudder with a the prop a large distance from the keel and rudder. (photos are available but will confirm this) It is a 15x9 prop. Engine is a Universal M35 4 cyl 30 hp engine 1.8:1 reduction. Mounted on the center line. Rudder is perpendicular and is a balanced spade rudder. Boat goes straight under sail with no wheel pull.

Former prop was an older model 3 blade feathering prop. I sold it since I would stay with the better performance of this prop even with the problem, but I am not certain if a prospective buyer would agree. The new prop gets 2950 of 3200 rpms. no smoke, no overheating.

I now get 6 kts at 1800-1900 rpms, with a noticable pull on the wheel to the left (with the boat wanting to go left). At 6.5 at 2300 rpms the pull is very considerable.... In either case if I let go, the boat goes quickly to the left in a tight circle - hardly a safe situation. These rpms/speed are about 1/4 to 1/2 knot faster than before

BUT the powering performance is truly superb - I would not trade it.. and of course when racing the folding prop is as expected. There is no noticable prop walk in reverse and the stopping ability is quite sufficient. In general a very superior folding prop.... except for this one problem. The manufacture has been very helpful but has not run into this before after many sales worldwide... although the problem in general is not unheard of. He says various subtle factors can be working... placement of the zinc, distance of the prop from the boat fore and aft, etc.

So he is supplying relpacement 15/8 blades (just have to replace the blades, not the core) which will get the rpms up to 3200 max and hopefully will help. But it requires a haul out because of the use of duplicate set screws using loctite as a final backup. It will be next year before my normal haulout and I would pay for a haulout now if the problem could be resolved. (my wife is not fond of holding the wheel under pressure while motoring)

I suppose my concern is that with a reduced pitch, I might get the same degree of pull at the same speed as before (while at a slightly higher rpms). Since I would like to go 6 to 6.5 knots, there might be no real changed in the pull. Or will there be a change??? Or would a reduced diameter be better... as my local marine mechanic says.

One Internet resource indicates that with power boats at load speeds, an over pitched prop will cause the boat to turn. But that is mainly a power boat web site.

[Idiens]

I suppose a two bladed prop could have a more pronounced effect than a three bladed prop when absorbing the same power.

I presume the effect is caused by the flow from the propeller being a spiral and hitting the rudder at two different places, high and low, in opposite directions - from a centre of pressure point of view - i.e. the left turning moment is greater than the right turning moment on the rudder.

A skeg would perhaps help reduce the effect. With a balanced blade rudder, one would need to correct the balance with regard to the prop flow. That implies a new rudder design.

[retclt]

Originally Posted by sailingdog

It would help if you said WHAT PROP YOU BOUGHT. Someone on the board may have experience with it... but if you don't say what brand, make and model it is, how can anyone help you???

I highly suggest you read the post in my signature... the more relevant information you can give generally results in better suggestions. As they say in the Computer Science field... GARBAGE IN ===>>> GARBAGE OUT.

Lighten up Francis!!! Geez.


Adding a trim tab to the rudder would do the trick but when not under power you'd have to adjust. No advice here . . . I just wanted to tweak SD since I'm negative proof now.