boat is a surtees 7.3 game fisher. Yamaha F225 V6 4.2

thanks alot for the reply!

lovernanako wrote: thanks alot for the reply! but there is one problem! I ordered a new boat and wanted to choose the proper propeller before I get the boat. do you think that after I received the boat, I can exchange the prop with my boat dealer for another size? thanks!

my understanding is that it is onus of the dealer (surtees) to rig the motor so it achieves engine manufacturers stated maximum operating speed

thats not necessarily meaning that it is going to offer best boat performance though

whilst there is a science behind selecting a prop, the final optimum result is often requires some trial and error

1/ decide if for economical cruising, or top speed.
2/ Decide what cruising or top speed (see 1/) you would like
3/Decide at what rpms your cruising rpms u would like
4/establish the fully loaded weight of the boat.. gear fuel ppl etc.
5/Know the water line distance
6/ using Crouches formula (google) calculate the hp required to achieve this
NOTE: this is NOT the hp rating of the engine, it is the HP at the rpms at which u wish to cruise at... plus about 5% to 10%... so u need the power curve of the engine.

7/ u now need to know the gear box ratio.
8/ working on around a (cruising) 10 to 12% slip (thats the 'grip' the prop should have) knowing the rpm cruise, the cruise speed, and gear box ratio u put these into a forumla and it comes up with your pitch.
The slip or 'grip' is determined by , basically the diameter and number (surface area) of  blades... things like rake and cupping also influence between manufactures

u have a manufactures recommended top rpm range... with a engine that is only JUST MARGINAL(crouches formula above) then u have a slower less economical cruise if top of the range flat out fully trimed, fully loaded....
Ideally need to be about 100 rpms above the middle of that range... compromise between speed and cruise economy
 talking marginal HP engine to boat) the engine is to loaded up for longetivity .....so If say trimmed top rpms is 4700 and range is 4500 to 5500  dont cruise above around 4000K
Above that the gap between the HP requirement of the prop and HP out put of the engine is to small and long term reliability is an issue.
Have another min approx 20% engine HP

Read this several times and get to understand it
http://continuouswave.com/whaler/reference/propellerPowerCurve.html

U will be VERY lucky to get any professional advice other than to have a prop where can , fully trimed, on smooth water get to max manufacture recommended rpms.....regardless of slip factors etc.

Another thing that have noticed is ppl use L/Hr.... when I have put these numbers back to distance per L  regardless of L/ Hr at a given speed big differences.... the dist/ L remains near the same.

Professionally proped and then repop as per the above, gives me a cruise speed approx 5km/ hr faster at approx 1000 rpms lower with near 50% less fuel.....but not a good idea to open the throttle wide up... 2 reasons
1/ engine loading
2/ The increase in top speed over the 'professionally ' propped unit is just down right dangerious ....twitchy like a race boat WoT.

I calculated what I need ... trialed several prop that I KNEW where not what was needed to confirm any predictions.... could not get the prop I calculated because of cost... and compromised on a smaller 4 blade that in effect over did a little what I wanted.

That was the simplified version. Shot steps

Surtees being a deep vee love HP and the more the better as they sit low in the water
For example most people with 6.1 would go 150hp which is the max engine, and trust me you wouldn't want less than that on that boat. Same with the 5.5. You want 115 HP which is top of range.
Never been on a 7.3 but I'm sure the 250hp would be better

it was a revolution with a 17inch pitch a 15 might be better for you to give more oomph of the line with a flooding keel the 17 works well on my boat which is a similar size top speed is about 70kph

Agree with FA.  I have a Surt 6.1 and when re-powering would have gone with the Suzuki 175 if I could have swung the deal. Ended up sticking with the Yammy F150 and would not go any less.  The 5.5 also loves power and 115 seems to be the go for blue water work.  So the rule is  to go with max HP for the Surtees Hulls.

My F150 has a sweet spot in L/Nm between 3,800 and 4,000 RPM, I think the F250 would prob sit in the sweet spot better than the F225.

Be careful going to 4 blades....althu a 4 blade may have a lower surface area than a an equivalent 3 blade 1+" diam above it has huge more 'grip'
Yes have done that exercise myself as well....
Suppose that part of "rocket science"
If u can only get to manufactures bottom recommended rpm range... dont go above approx 800 rpms below that... the gap between the HP requirement of the prop and HP out put is too small and loads the engine  for shorter life.
Also run calculations(google) on your slip ....if that gets below around 10% the cupping and surface area, is so great it influences the effective pitch rating of the prop....in effect increases the effective pitch rather than a real slip factor.
You will still get the stern lift/ economy and mid  to cruise range manoeuvrability....but have excessive loads on the engine unless cruising.....

It was suggested to me by a very experienced member here to sort the grip ... before sorting the the final pitch.....and all my trial props have tried, the graphing , formula definitely confirmed that approach.