Hunter 40.5 two blade prop broke in half! Need new one.

[anchorclanker]

For every inch of prop diameter you increase, you cut 1.5 to 2 degrees pitch to maintain engine rpm. Your adding two degrees, so between the two your motor will see a much larger prop thats pitched to push the boat 13% faster. If it was properly matched before, you are going to be way overpropped.

 

[Dennisail]

For every inch of prop diameter you increase, you cut 1.5 to 2 degrees pitch to maintain engine rpm. Your adding two degrees, so between the two your motor will see a much larger prop thats pitched to push the boat 13% faster. If it was properly matched before, you are going to be way overpropped.

How will it be over propped? I had a 18D x 15P. New is 17D x 17P. BTW we are measuring pitch in inches here not deg?

New prop is smaller at 17D so going by your 1.5 to 2 of pitch per inch of diameter I can have more pitch and that is exactly what I have. The new prop is one inch smaller in diameter and 2" more pitch. If it is slightly overpitched I will be happy as mentioned previously in this thread.

If I chose a prop from new I would not have got a 17x17 but I paid one quarter the price and the specs appeared to be within the guidelines by the 1" of diam = 1.5-2" pitch rule.

The real test will be on the water. I hope it even fits on the shaft!

 

[anchorclanker]

I apologize for the mix up, but as you likely know, prop pitch gauges are calibrated in degrees, and actual measured blade pitch changes across the blade from root to tip, with greater pitch at the root, and less at the tip. The pitch stamped into the blade is the theoretical distance the prop would travel in one revolution at 100% efficiency.

If your boat was reaching near hull speed before, it cant go any faster. It just wont. Adding 2 inches additional pitch is the equivalent of asking the boat to go 13% faster.

But your losing an inch in diameter so one of two things will occur. Either the smaller prop will slip enough (loss of prop efficiency), or it wont, in which case the engine will be incapable of turning it at full engine RPM, overloading the engine.

But as we dont know how the boat performed previously, or how well the two propellers equate to one another in quality/efficiency, its hard to say how the new prop will perform without physical testing.

Further, Marine Diesels, unlike marine gas engines, are governed. The max engine speed is locked in and even without any prop at all, diesels wont exceed the maximum no load speed. From that point, the only thing that would pull the speed down, with the throttle fully forward, would be too much prop. The goal then, is to find a prop that allows the engine to reach as close to maximum speed with the throttle fully advanced, as possible. Anything that pulls the engine down is hard on the engine. Less prop wont hurt the engine unless you go way under, you just wont get the full rated HP available.

If I were looking to increase low speed thrust, I would go with a larger prop and less pitch, not smaller prop with more pitch. It would sacrifice maximum forward speed as a trade off, but the prop would be much more efficient, and the engine would run with less load.

However, larger props run into clearance and sound/hull resonance issues. This is where the three and four blade prop comes into play. A really good prop, like a three blade Campbell, could actually allow you to decrease pitch further, possibly to 13 or 14, yet still maintain current forward speed (through greater propeller efficiency) while increasing low speed thrust.

 

[eianm]

Dennisail- i have a 1991 H42 - i upgraded to a 3 blade variprop and the improvement was amazing- you really must try to go for a 3 blade folding - if at all possible budget wise- you WILL be pleased with the outcome!
In fact, if my budget had been bigger, with what i know now i would have paid for a gori- the overdrive function is amazing- a friend on our marina has one on his Jenneau 42 DS- WOW!

 

[Dennisail]

AnchorClanker. I totally agree with all you said in your last post regarding prop sizing. Rick went one size larger with less pitch, which should be more efficient as big props have less slip.

However I went the other way with one size smaller and more pitch, not because I thought this would be the ultimate combo, but because this prop happened to still be within the rule of thumb guidelines. This prop just so happened to never have been used, yet was selling for 1/4 of the price of this exact prop when new.

My theory is that with a clean bottom motoring with the wind, and particularly when motor-sailing, the extra pitch may more than make up for the loss in diameter, because the engine RPM may be less in those situations because of a low prop loading. However when the prop has to work hard against wind, chop and a dirty bottom it will slip more than a bigger prop with less pitch causing a loss in efficiency.

Either way I will finally be hauling my boat after an insurance nightmare. So we will be able to find out soon if all goes to plan. I think we call all agree the sailing efficiency will be far better and it is a sailing boat. I sail my boat rather than motor it

 

[Steve Dion]

If you are staying with a fixed prop, consider a Campbell Sailor.

 

[Dave Groshong]

http://shop.hunterowners.com/hp/part.php?m=40.5&c=12&p=52261

 

[Dennisail]

Thanks guys but there must be some confusion. As I mentioned earlier I already have a new flexofold geared folding prop I picked up for a great price in the USA and Rick was so kind as to ship it out to me.

 

[Dennisail]

So the boat is out and I tried to fit the new prop. The hub fits perfectly but the thread for the nut is way to big to fit the smaller nut that comes with the fexofold. I cant machine the nut out because there would be nothing left.

Looks like I have to get the shaft machined to the smaller thread. I just found out the shaft is just a little too long to be removed with the rudder in place! I got it almost all the way out. Oh well there is some movement on the rudder shaft and some new rudder bearings should be on the cards.

What sort of price would rudder bearings usually cost? Should I buy some from Hunter or just get some made up in Australia? I cant find the bearings on the hutner site, only a whole rudder.

 

[fstbttms]

So the boat is out and I tried to fit the new prop. The hub fits perfectly but the thread for the nut is way to big to fit the smaller nut that comes with the fexofold.

This doesn't even make sense. The prop is the correct size for the shaft diameter but the nut that came with the prop is the wrong size? You bought the prop new, yes? If so, why not have the seller send you the correct nut? Or is it possible that you do not actually have a 1" shaft but rather a 1.25" shaft and purchased the wrong prop?

 

[Allan12210]

Can't you use your old nut?

 

[fstbttms]

Can't you use your old nut?

The Flex-O-Fold uses a special barrel nut. The nuts that secured his fixed-blade prop are not compatible.

 

[Dennisail]

No its not possible I purchased the wrong prop (well not entirely). This should be obvious as I said it fits perfectly. Yes the shaft is 1-1/4, which is why I purchased a 1-1/4 prop. I said 1" back in my first post, but established it was 1-1/4 pretty quick.

As mentioned in the thread, the prop was "second hand", but appears to be unused. I got it MUCH cheaper than a new one.

The nut that came with it is too small and will not fit my shaft thread as stated. If I bored it out, the part which the socket fits onto would get too thin, if not non existent. So I would not be able to tighten it up.

I can barely see how any nut can fit in there since the inside diameter of the hub is not all that much bigger than the shaft thread. The thread appears to be 7/8 and the hub ID is 1-1/4 leaving virtually no room for any sort of nut.

I am removing my drive shaft and getting it machined down to fit the nut. There may be another option. I did email flexofold but no response. Perhaps there is a bigger hub (with the same size taper and shaft) that has room inside to fit a more sizable nut. Even if this were the case it will be far more expensive than machining the thread smaller which will cost about $80.

Also, I am doubting the specs of 17 pitch. I don't really know how to test it, but it looks no steeper than the old prop.

 

[fstbttms]

Well, something is amiss. Flex-O-Fold make props to fit every common shaft diameter (be they SAE or metric) and there ain't nothing special about the shaft on any Hunter boat, so either you bought the wrong prop or the nut that you got with the prop belongs to a different size prop.

Assuming you have the right prop, common logic dictates that simply buying the correct nut from Flex-O-Fold is the proper fix for this issue, not modifying the shaft to fit the nut you have.

 

[bananabender56]

If the rudder on the 40.5 is same as the 43. (1991 vintage) then the play in the rudder could the housing that the bushes go into is worn. I had bush movement within the housing with no wear on the bush ID. The bush was then locked into place and the slop diminished.

 

[Dennisail]

The flexofold is on and it is the right prop. Just needed a smaller thread machined on the shaft to fit the flexofold nut as I said. The rudder has been rebuilt too with a new shaft which was a few mm bigger so now there is no slop there either.

Top speed is 0.2 of a knot slower at 7.8 rather than 8k. But would be able to do this all day as the engine is underloaded slightly not overloaded like with an oversized prop. low rpm speed is the same if not faster as it has more pitch and more blade area. Reverse is not as strong and the prop walk is greater as expected.

More importantly with all the cruising gear removed and a new bottom job we kicked ass at the local Wednesday afternoon races where previously we had done badly. Even the upwind performance is decent and it points higher and will less leeway. It does 7k @ 30deg. The boat is MUCH faster.

This is such a great boat. I might regret selling it one day.

 

[Rick D]

Races

Well Dennis, great job and good results. Congratulations sorting out all the insurance and repair issues relating to the attempted theft of your boat by that felonious whale.

Hey, did you replace your A sail and did you race with it? If so, it might be a good idea to let these guys know about the need for a jackstay to reduce the upward pull on the anchor roller. (Well, never mind; I guess I just did )

 

[Dennisail]

Thanks Rick. I did not replace the Code Zero as I am selling the boat anyway. So I did not not add a solid bob stay. But I would recommend it to anyone hanging any sail off the end of the anchor roller. A rigging wire or high tech rope would do the trick too and that is what I done half way across to the Marquesas. But I think a thick wall stainless tube would be the best as it will also take care of downward forces which can be large when trying to free a stuck anchor in large swell.