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we have kiwi prop and last monday off park pt,incoming tide sw 25+ at times 16hp yanmar 3/4 throttle  at times 0.5kt forward,normally in flat water would of got 5.2 on clean bum.So is it the prop not biting in a sea way? first time in a sea way for me in this yacht.

I'm no expert on props in seaways etc, and would tend to agree with Wheels - 25 knts head wind and a seaway is a lot of additional drag in relation to what is needed to get up to 5.2 in calm flat water.

What I did want to say though, is that kiwiprops do require a fair bit of maintenance. Performance can drop off if they are not in good condition. If the boat (and prop) is new to you, it may require some love. There are the obevious things like fouling stopping the blades going to the correct angles, this can really screw things up. We found a barnacle in residence on one of the stops once (but we did dry out because the prop performance was bad and we were fairly sure it was badly fouled. 

 

There are other little things like wear on the blades around the stop areas, or the stop pins getting worn, or bent. That can happen if you jam in in reverse a bit too enthusiastically. All of the components are are easily replaced on the kiwiprop, and most of them don't cost much, but they do wear and it could affect performance. You would notice it in flat water as well as a seaway though, accept it may be more pronounced in a seaway.

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Harry and wheels the Cav 26 is or was a quarter ton class, relatively heavy for its length and short on the static waterline probably about 20 to 21 ft but may pick  up more length as the stern wave moves out towards the transom (effective waterline). These boats were renowned for pitching. Theoretical max hull speed just under 6.5 knots 16 hp should be more than adequate to achieve this. My feeling is there is something wrong in the prop selected or drive train. 4,400 lb displacement although this was in racing trim, from memory Ron Holland was the designer. Certainly worth getting Henleys to run it through their computer.

From memory these boats had to have quite a steep shaft angle which would also rob efficiency

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The only feathering props I have had anything to do with are the Max prop which was a two blader,(also available in 3 and 4 blades). These appeared to be more efficient than a folding two blader and virtually the same as a fixed prop going ahead but better brakes than a fixed prop going astern. It is pretty well known that the folders such as Briski's partially close in reverse so reverse performance may be adequate in a small light boat but no brakes on heavier boats. The best of the folders appears to be the Gori. I once worked on a 32ft Shipbuilders launch where the owner decided to import a prop from Australia which was a five blader. The blades were almost finger like in profile and top speed went from 8.25 to just on nine knots. Unfortunately this type of blade showed several disadvantages the first being no punch in a sea way and the second was even worse, no brakes. The first attempt at picking up the mooring resulted in overshooting the mooring, lots of noise and the boat running up on the closest mud bank.

Harry if you know of someone with a hand laser rev counter  get him to check the revs on the propshaft in relation to the engine that way you can rule out a gearbox problem.

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Harry, the Cav 26 does tends to pitch a fair bit with its fine ends so loss a lot of forward momentum (I had one called Panic Mouse for several years). Several times had quiet a job getting through the Motuhie Channel and ended up sailing which was a pain short handed and short tacking  as she needed a powerful genoa. The kiwi prop is unlikely to be the problem which I presume is on a 2gm20 (16hp). My Raven has this set up and has good drive as doesn't pitch due to its big bum.

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"Muzza" the cav 26 is a doug peterson designer,bum is clean so is prop.went out for a motor today and she pushed well in 25+ flat water though she does hobby horse but nothing i can do about that as she is 19ft waterline so there lies the problem

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Harry I always got Doug Peterson and Ron Holland boats mixed up probably because of the fact when Doug started his design career off on a boat called Ganbare he had a young Kiwi boatbuilder working for him called Ron Holland. You can still  see a lot of the Doug Peterson influence in Ron's designs. 19ft  static Waterline and probably pick up another 3 ft in effective waterline.

You are correct heavy, short waterline boat with fine ends equals pitching. With that amount of power you should still be good for  6.5 knots. a friend has a wright designed tracker with a twin cyl Nanni saildrive in it 13.6 hp and that has top of 6.8knots and will cruise all day at 6 knots on the smell of an oily rag.  

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Hmm, 19ft waterline gives a 5.8 knot hull speed IIRC. I would not expect to exceed that under power, at least with a 16 hp engine. Even if it does extend 3ft, that's about 6.4 knots max.

If it gets the revs in flat water, and achieves these speeds, without odd vibration or noises, then I'd expect the windage and sea conditions simply overpowered the drivechain. Many small yachts won't motor well into a chop and 25 knots +.

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one thing of interest is Water density. During the second world war PT boats stationed in the Aleutians had propellers two inches smaller in diameter than what they had been using in Hawaii. A thing to remember if using information obtained from a cooler or warmer climate.

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island time remember these boats pick up about another 3 ft of waterline when the stern wave picks up.The static waterline is only a painted line on the hull and is irrelevant in computing speed. It is the effective waterline you should be considering.

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Harry how does the shaft line look compared to the waterline from memory some of those boats used quite a steep shaft angle?

My last yacht I kept the downward angle down to 3 degrees but once you get over about 6 degrees a lot of energy is wasted in trying  to lift the stern of the boat out of the water. 

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The cav 26 looks totally different, for its time more performance orientated. The 25 stands out as a Salthouse design, for once Boat Data has it wrong. I think where the confusion comes from both were made by Salthouse's later Cavalier yachts.

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yep that's a Cav 26. Sail drive makes sense on that boat moving the prop at least 6 ft further forward than a shaft installation. Unfortunately sail drive manufacturers don't put low enough reductions on for engines running to 3,600 rpm Yanmar 2.64:1 Nanni and I think Beta 2.38:1 and Lombardini 2.18:1. a 3:1 would be better suited get max prop revs down to 1200 and  cruising down to 1000 or just over. 

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both designs become obsolete in a very short time as 1/4 tonners,had a 25(tachyon)with 10hp bukh shaft  2 bladed

imo the 25 has more room but the peterson sails a bit better.for the price they go for.both make good cruising family safe boats

i got it wrong it the salthouse 19.6 waterline and cav 21 waterline.

 

If I wanted a vessel to motor around in i would of brought a launch :razz:

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Harry. As has been stated already most auxilary installations are a compromise and often props are not that well matched to the engine your 14 x 8 does seem to be a little short on pitch for a 16 hp engine. The standard reduction on those Yanmar gear boxes (Kansaki I think was the maker, or some name like that) is 2.2:1. A mate has a Raven 26 with with an identical engine to yours Yanmar2GM using a Briski  folder is getting top speed of 6.25 and comfortable cruising at just over 5 knots  and that boat is probably at least 500kg heavier than your boat.

A 21 ft static waterline would mean an effective waterline of some where around  23.5ft so  a max speed of 6.5 with a properly matched prop. Things like the pitching you can do very little about , it is just in the nature of the beast but I think your installation could be better matched to the engine

The worst installation I have seen was on a 32 footer powered by  a 10hp Bukh in a flat calm after the alignment problem was sorted this boat would wind up to 5 knots on 29ft wl but on launching vibration through the entire rev range was so bad the engine could be only used at idle. This vibration was so bad it gave you a headache after only a few minutes. I got the owner to take the engine up through the rev range in neutral and no vibration was apparent. We decided the boat had shaft alignment issues so went home and bought down the tool box. I decided to start the alignment from scratch and on slackening the the bolts on the front engine mounts to check for any movement and straight away the engine started lifting off the mounts and ended up floating about 3 mm above the mount. After half an hours perseverance the alignment was finished and the boat could be taken up through the entire rev range without any vibration issues. I was later talking to one of the apprentices who worked on the boat during construction and he said #### the owner of the boat building company had  an attitude that Yachts are meant to be sailed and engines were just basically thrown in and bolted down, alignment was not rechecked after launch although in this case with the boat being cold moulded  the shape of the boat wouldn't have changed that much.

I just had a look in some old books and the Salthouse 25 design was earlier than the Cav The Salthouse 25 was about 1973 vintage where I think the Cav 26 was 1977-78.

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my little 25' keeler had a 16hp engine with 2.5:1 reduction gearbox 

 

weighed more than 25' cav/salthouse/reactors with more wetted area, (almost full keel) and only 20' waterline

 

subject to pitching as it was a narrow double-ender

 

it had a 3 bladed,15 x 10 prop which would get it pretty easily up to 5 knots at medium revs, no rev counter

 

taking the revs higher in flat water didn't make it much faster, it just screamed and towed a big hole + wave behind the boat

 

https://youtu.be/quImf233by8?t=29

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One thing I should have mentioned in calculating propeller size is that prop shaft revs is one of the most important things to take into consideration. An engine developing 10hp at 3000 revs on a 3:1 reduction is virtually the same as a 10hp at 2000 rpm through a 2:1 box, all giving around 1000rpm on the prop shaft. Another thing that people often don't take into consideration when selecting props is the horsepower soaked up by other auxilary devices such as engine driven compressors for fridge/freezers or Aircon units, these can easily soak up 5 to 10 horsepower best to have a magnetic clutch on them to turn them off when not needed.

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