Prop opinions

[Spiderpig]

I have just bought a boat that I have been chasing for a couple of years. Its a dolphin flexihull, a poly longboat.

It came with a yammy f60 which I wasn't happy with, so I took the yammy off and put my 4S merc 60 bigfoot. After going for a couple of runs over the weekend I am happy with the merc, but I will have to change props.

The boat is 6.2m and quite heavy:

Hull 680kg
Engine 120kg
fuel 60kg
Max 8 passengers
misc stainless, 200L esky, bait, booze ice, fish

It has got to be pushing 1200kg. I knew it was going to be a slow boat, but I bought it for the ride.

At the moment I am running a merc vengeance 13-3/8" x 14P prop. I am getting 22mph at 5400 WOT. The setup is too heavy to use the merc prop selector.

I have tried the motor at one hole higher but it was cavitating and the cav plate was out of the water. It made no difference to WOT.

It also has a SE foil on it which I plan to take off to see what difference it makes.

Ideas?

[Moonlighter]

No, other than to talk to Steve at Solas props. He will have some options for you.

[Spiderpig]

Yeah I will be calling him tomorrow. I actually prefer merc props to solas props but the ability to return props is probably going to push me to buy solas

[Spiderpig]

Steve from solas couldn't help. Back to the drawing board

[Moonlighter]

Are you a member of The Hull Truth? If so put a post in the boating forum there and ask Ken from the Propgods to reply. If he cant help, no-one can.

[Chamelion]

6.2m poly with a 60hp.. surely you just need to go a bigger motor? I have a 40hp on my 4.1m tinny and when loaded with a couple of people and fishing gear I wouldn't like to have any less ponies available.

[Spiderpig]

Max hp is 70 on these and the only 70 worth buying is the suzi. I still don't believe (and Steve agreed) that I will gain much from the suzi 70.

I am starting to think that it isn't the prop. There has been 2 pods welded to the boat that are flush with the bottom of the hull. I have a suspicion that these are acting like big trim tabs. If my theory is correct and these boats need to ride with the nose high, then I could stand to gain by removing them.

There is another member on here with a suzi f70 on exactly the same boat that is getting 33mph WOT. There has to b something else a miss.

[Spiderpig]

For anyone thats interested, I cut the pods off yesterday and gained 3mph at the same revs.

Going to try a motor wedge and removing the foil before playing props.

[BigE]

IMHO
longboats were designed to lift load with low HP. it will never be a "fast boat" I would be looking at the motor height and then prop, have a look at the spitfire four blade from merc. it was designed for good fuel ecom at cruise speed and should compliment your style of hull.

BigE

[MTAQ/BTAQ]

My Flexihull has a cut away transom and so the motor sits lower - does yours have a cut away transom?

[Spiderpig]

IMHO
longboats were designed to lift load with low HP. it will never be a "fast boat" I would be looking at the motor height and then prop, have a look at the spitfire four blade from merc. it was designed for good fuel ecom at cruise speed and should compliment your style of hull.

BigE

Totally agree with this whole comment. I knew it was a slow but MTAQ is getting over 30mph and I am trying to get the boat setup properly. I don't believe it has ever been set up correctly.

I got online to ask opinions about the mercury spitfire pontoon prop 13.8 x 13P, when I noticed your comment; I thought the thread was dead. I have pretty much decided on this prop after spending a lot of time reading the seppo forums. There are plenty of them getting high 20's in mph with the 60 bigfoot and 20ft pontoons.

The big question is do I do 13P or 12P?????

If I only get 100 -150 rpm out of a 13P (down one size) it I will only just be hitting WOT which is 5500 -6000. Should I be going to the 12P and aiming for higher in the WOT range?

Anyone know what the effects of going down one pitch and to a 4 blade will be?

[Spiderpig]

My Flexihull has a cut away transom and so the motor sits lower - does yours have a cut away transom?

Yeah Lloyd, same transom. I did my research on flotation and it turned out the pods and foam within weighed more than the water they displaced, They were actually adding weight to the transom.

[FNQCairns]

"They were actually adding weight to the transom"

How can this happen? if not waterlogged?

The rolled gold rule of thumb in proping all boats that plane but still have the prop shaft speed and engine height of any production fishing boat is.....highest pitch possible combined with the lowest diameter and the least number of blades on the prop.

Short of the bling factor and marketing or community belief systems this is the only best case scenario for typical planing fishing boats.....every fishing boat....twin hulls do change the equation slightly...but only slightly.

[Spiderpig]

"They were actually adding weight to the transom"

How can this happen? if not waterlogged?

The rolled gold rule of thumb in proping all boats that plane but still have the prop shaft speed and engine height of any production fishing boat is.....highest pitch possible combined with the lowest diameter and the least number of blades on the prop.

Short of the bling factor and marketing or community belief systems this is the only best case scenario for typical planing fishing boats.....every fishing boat....twin hulls do change the equation slightly...but only slightly.

The pods were made of, 12mm poly and full of flotation foam for some reason. They weighed more than the amount of water they displaced.

I'm not sure about your golden rule, I have spoken to Steve at solas, Ken at propgods, 2 mercury dealers and spent countless hours reading. No-one has told me to go to an 8 inch 2 blade prop. Thanks for your opinion though.

[MTAQ/BTAQ]

I can't understand why there would be 2 "flotation pods" OUTSIDE the hull which create drag and weigh more than the water they displace, my Flexihull has 2 pods INSIDE the boat at the rear disguised as seats either side of the motor maybe they are trying to replace these?

[Spiderpig]

I cant understand either mate. He had plastic welded them on to the back for some reason. Why you would fill will with foam is beyond me. They would help the boat stay a float well with the bungs out.

A picture speaks a thousand words. You can see the waterline in the photo, there is not much of a very large heavy pod in the water.

The photo doesn't show it well, but the back of the pod was definitely lower than the line of the rest of the hull; creating drag. Pods like this can also cause prop disturbance if poorly designed.

[Andrew Wylie]

Hi,
have you gone back to where you purchase the weapon from, and asked any question or get any history on the weapon and why he/she did what he/she did, and then go from their.

Just a thought, it might help you get your head around it.

Regards
Wheels.

[Spiderpig]

Ive worked the pods out, they were unnecessary. I have changed the engine and now need to work the prop out. I have just got hold of a vengeance 12P to try next weekend!

[Spiderpig]

Ran the 12P vengence today. 2 blokes, 70l fuel, ice, carton of beer and fishing gear.

Got 28mph @5800rpm = 1% slip - I am now reaching WOT.

Only got around 18-19mph @4800rpm =24% slip.

From what I have read, props are designed to run the least amount of slip at their maximum speed.

Will I get a better cruise speed with a four blader?

The mercury spitfire four blade only come in 11p or 13p? Which one should I try?

[odes20]

Those pods would certainly bury the nose, whoever built them both heavy and in that shape had no idea. Youre propping search is exacerbated by trying to get the max performance out of an underpowered donk in reality. I think a 70 propped correctly would make the whole rig a real joy to operate.

[Darren Mc]

When you go from a 3 blade to a 4 blade generally you need to go down minimum of 2 pitches to stay around the same wot rpm.