# Conchfish Prop



## Sublime (Oct 9, 2015)

If you can get it to run at negative trim, then I think there's a chance you can get a prop with a lot of cup to run at normal trim.


----------



## ElLobo (Sep 14, 2020)

You could try a bow lifting prop.


----------



## coconutgroves (Sep 23, 2013)

My Whipray has to run at negative trim or I have to tab the hell out of it. My concern is that you may start to porpoise if you change the setup, but without pics it is hard to say.

Tagging @Chris Morejohn to see what he thinks.


----------



## jonny (May 1, 2007)

If I remember correctly Tohatsu is set for a 16” transom for their 15”. So you are not too far off. I did my transom at 17”. But I have a jackplate mounted a bit lower for my 15” 50. Don’t know if you were wanting to go with a jackplate or riser. But it would solve your issue and be beneficial for other things as well


----------



## mwolaver (Feb 6, 2014)

Manual jackplate will give you a setback.


----------



## Chris Morejohn (May 12, 2014)

Joekurimai said:


> Looking for some more advice. I built my transom at 17.5” for some reason on my conchfish and the dealer installed the MFS60 short shaft. They installed on the lowest hole and motor is slammed on the transom. Right now at neutral trim the cav plate is 2” above transom. Aluminum 11x15 cavitates at anything other than negative trim, motor tucked, which has the boat in stuck in bow down position running. My question is it worth trying to find a stainless prop to try and run properly at that height or just cut, fill, and redrill transom? Speed is more important than holeshot to me. Thanks


The cavitation plate when trimmed down fully can be 3/4-1” above the bottom max for fixed position.if using a setback or jackplate then you can adjust to suit.
My designs use trim tabs as part of their trim.
I suggest to trim tabs all the way down at idle. Trim engine fully down. Get up on plane and adjust tabs so the hull is just trimmed to not porpoise at speed. Then use power trim to run bow up or down. Get a feel for this, then when yo re running in different wind and sea conditions you will have a feel for the trim.
If running down wind and good chop you want the tabs up and be able to trim the engine up and down to suit the bow angle.
Beam seas the hull can be tilted to suit the spray. Most times it’s best to angle down into the wind to keep the outgoing spray downward. 
it’s best to build the sterns to my design specs.
If using a short shaft engine never build the stern height above 15”.
If building to a 20” shaft never build the stern above 20”
Having the engines motor mount bolted an inch above the stern is fine. The bolts are what holds the engine in place.
The engine is pushing the hull around thrusting from the bottom bolts forward and the top bolts are holding it from tilting aft. 
if the deck and stern are all bonded and built to spec then all is well.
Use different props for different needs,not to trim the skiff for all round use.


----------



## Joekurimai (Jan 9, 2015)

Thanks Chris and everyone, right now with the motor tucked full down the cavitation plate is 3/4” above the transom and planes instantly without tabs and rides satisfactory at low cruise 25mph or so. The faster I go the more the bow digs and usually working the trim up and tabs carefully through porpoising I can get close but will cavitate before neutral trim (cav plate parallel with the hull.) At wot 39mph I can feel the whole hull digging in the water, super squirelly. 

Anyways I’ll prob try a prop with a slight cup before cutting and drilling new holes or maybe the setback plate route. Any recommendations for prop? Thinking of starting with the SCD. Thanks


----------



## Sublime (Oct 9, 2015)

39 is hauling butt !


----------



## omegadef (Jul 10, 2011)

My boat is not a conchfish. I'm running the same engine with an SRD 15 pitch. I lost quite a bit of holeshot from the aluminum prop and only get 5krpm now, so I ordered a 14 pitch but will keep the 15 as a spare. My top speed is 30-31 MPH. You are *scooting* at 39mph. Do you know actual rpm? IIRC rev limit was 6300rpm which would yield like 9.3% slip at your numbers, anything less than 6k rpm would be suspect. 










My boat is much bigger and heavier, but for comparison sake I have 12% slip at current:









With the 13 pitch aluminum I was at the same speed, but on the rev limiter, so the slip was 19.5%. The cup/rake of the SRD seems to makes a big difference.


----------



## Joekurimai (Jan 9, 2015)

Working on getting my chart plotter hooked up to the engine so I don’t know RPMs yet. Speed was from the phone so could be off but it feels fast and if I can get the hull out a bit prob faster. Engine is singing though but no rev limit. Hull is light tho, everyone stares when I pick it up and carry it halfway up the beach


----------



## coconutgroves (Sep 23, 2013)

SCD and SCB are both stern lifting props, this means you will get stern rise out of the prop, which will make the bow come down. I think that would make your issue worse. I tested the SCB and did not like the performance - did not jump on plane as good as my Foreman prop. I am in a 2015 Whipray with a Tohatsu 50hp short shaft. Not too far off from your rig.

Jack Foreman makes a mean prop for your engine and also makes larger water intakes that work great with the compression plate.

How much weight to do have in the bow?


----------

