bertram31.com

From searching the old board the consensus seems to be for our B31fbc Yanmar 315s 2.03:1 ZF63a that the props should be 4 bladed 20" x 23".

Any comments?

We expect to be at the lower end of weight estimates, early hull but no generator, air, desal or tower - a day boat with teak cockpit & moderate fitout but almost no "stuff". Beer, a few fat boys but skinny girls.

Suggestions on cupping & tweaking welcome.

Thanks

Nic

Nic,

It sounds like your boat will be very much like mine: bimini top only, no extra heavy equipment, etc. I have the four-blade 20 X 23.5 (approx, as there is some pitch difference between port and stbd due to the reversing gear on one side). The four blades are very smooth, and I get plenty of speed (max rpm = 3950; 30 kt cruise @ 3600 rpms; 26.5 kts @ 3200 rpms).

Tommy

Ditto on what Tommy said. Only difference is I have my props pitched for 3850 WOT, not 3950. Other numbers are exactly the same. I have hull 205, 31SF with the 315s, no "stuff", no bimini, not even skinny women (note to self: get skinny women for 2007). 20x23 is probably a good starting point, but you should definitely get your prop guy's opinion.

Bill B wrote:not even skinny women (note to self: get skinny women for 2007).

Good one Bill!

Is the pitch different with the skinny women compared to the fat ones

Mr Minton,

A very different pitch and higher maintenance too

Nic

Definitely higher maintenance. Dave K

i think the pitch is the same but the CUP is usually less!

Guys....guys, settle. We're trying to sort out the props remember!

Nic

Hopefully I will be testing by the end of the year . Yanmars 6LP @ 315hp. Mastry sent a set of 19X23 to start with.

That's interesting, when I left my engine/prop guys to their own resources they suggested 18 x 24s which I have over-ruled based on the information here. My guys tell me they have been getting better than factory results for all their other engine/prop installs but none on B31s.

But I just can't see 18 x 24s achieving a better result than 20 x 23 (or 23.5s)

Any comments?

Nic

Nic,

Sometimes the smaller diameter is necessary to maintain the minimum clearance between prop blade and hull bottom, which I'm told is 10% of the diameter of the prop. My 20" props clear my hull by 2'', so I'm right on the line. Obviously, shaft angle impacts the clearance issue. Ideally you would test several different setups by trial & error, but that is time consuming and expensive. Let us know where you end up, and please give us a performance report.

Tommy

Thanks Tommy,

We've got the aftermarket struts from Hightide which for 20" props look like they'll give us exactly 10% tip clearance mounted straight onto the hull same as what it looks like to me on Buddy Boy. I would have liked to recess/flush mount the strut base but we'll fine tune it for clearance/shaft angle on the day.

We are still just getting all the parts ready in a box for haul out Dec. 4. I'll certainly let you all know how we go.

Props seem to be sorted, we'll go with 23.5" pitch, no cup.

That was actually very funny Bob, my stanadard pitch of "would you ladies like to go fishing?"works less as I get older, which is probably a good thing for all concerned. I like that idea of a section on the site for 'Ladies who like Fishing'. Capt?

Thanks guys,

Nic

nic if you fair the strut plate you could achive almost the performance gain with very little work.i ground to bare glass just in front of the structs,than built up with epoxy to form a fillet sanded down an eliminated the 1/2" step from bottom of hull to edge of strut facing the bow.

I have the same setup as Tommy. My boat tops out at 34.1 knots. I usually cruise at 26 to 28 kts., as per sea conditions. My boat is a 75. Some of the older boats weigh more, but they do hold less fuel, so everything may become equal when fully loaded.

Just ordered the props today at 4 bl 20 x 23, no cup. My guys are concerned they are a bit "big" (?) but we'll take that on the chin. It's a good starting point and as the engines are rated at 300hp here because of a lower octane fuel being the national standard for diesel we'll keep the half inch of pitch for fine tuning after testing.

Naturally we're now looking for a local supplier of the higher octane diesel.

Thanks for the input, we'll let you know.

Nic

I have a performance question. After reading your posts I wonder. I have bigger rudders than the stock 28 rudders. I noticed the leading edge is quite wide and blunt. Will that have negative affect on performance? I thought about grinding them to reduce turbulence from the prop but never got around to it. Does the modification to the strut mention in an above post help increase speed? Every little bit helps.

Nic,

Use Stanadyne Performance Formula Diesel Fuel Additive both for C-tane boost & fuel conditioning... (C-tane is for diesel, Octane is for gasoline.)

Kevin,

The leading edge of the props should have no more than an 1/8" of thickness...

Br,

Patrick

Kevin,

I don't think any performance change could be measured, as Bruce said on another post these aren't performance boats. BUT, we just want it to be as good as we can make it and fairing in the strut base can't hurt. Might give us a phsychological edge over.....um....., I can't think of anyone at the moment, but if I'm ever losing an argument at the bar I'll mention it.

Nic

Pretty sure mine are close to half inch at the top. They are thick!

BUT, we just want it to be as good as we can make it

Nic,
If you follow that logic then what you would want to do is recess the whole strut plate into the hull. The drag on a face fair is less than the drag on a flat face but there is no drag if the plate is recessed.

See how far it can go if you over think the job on a project where effort far out weights the gain.
It would be like porting and polishing (fairing them out) the heads on the family SUV.


In perfect sea and weather conditions you might gain 1/2 to 1 knot by fairing everything on the bottom down.
High Tides struts have been known to slow a boat by 1 knot.

But its not needed unless you have time not devoted to anything else like drinkin, sex or watchin sports. And in my 48 years of livin I ain't found one person yet that would rather work than do one or more of those items.

Kevin, your out on the water pretty much all the time. How often would a half knot at wot on the 28 be usefull and tolerable?

Again these ain't race boats where every 1/10 of a knot makes a difference or are they designed to run that fast in most seas without turning your kidneys into soup.

Save the fairing for above the deck.

It's just a boat, its not the space shuttle....................

How often would it be tolerable.....hmmmm. I never turn down a race, especially if it is a Pheonix or another similar looking boat. I can put a TV on the boat, that covers the sports. I believe I can put a lot of beer on the boat, that covers the drinkin. They don't call it a stabbin cabin for nuthin. Throw a grinder into the mix the next time the boat is hauled and now you have a party. Why not some clay pigeons and a 12 guage? Yes, I suffer from an overly creative mind. I am dying to hear a reply on this one!

Bruce,

That's what I was thinking, recessing the whole base plate. The fairing would have been a side bonus as the main reason was to reduce the shaft angle which I have noticed does make a difference. But looks like tip clearance won't allow recessing but having mentioned it to the co-owners I've created a rod for my own back 'cos now we're all talking fairing the struts up at the pub, even the barmaids!

I was on the sidelines in the building of a series of 45ft & 56ft Ray Hunt lookalike hulls and whilst we did put blanks in the mould to recess all the underwater fittings & base plates etc., bottom line, there is no substitute for horsepower. We couldn't quantify the speed difference the fairing made but the owners liked it. They were all in the 32 - 39 knot range, Aerolam F-Board internals, ducted combustion air, one had S&S blueprinted 1292s painted duck-egg blue which the chippies christened wood-duck blue. You get the picture.

Now, to put the cat amongst the pigeons, we DID notice an extra 1-1.5knots in the 35knot area between rolled and sprayed antifouling paint. I wasn't planning to spray but now you've questioned my sanity we are definitely spraying. There is a B25 Open on the Harbour we have to beat to the fish markets and the JetCat ferries on the Harbour cruise at 30-32knots and we think it's a good look at least pacing them.

Capt. Patrick

Thanks for the Cetane & additive tip.

Kevin,

I'm not going there.

Nic