Prop clearance

Rockasauras

Captain
Joined
Nov 9, 2013
Posts
856
Likes
298
Location
OCEAN CITY
Not sure this question belongs in this section but here goes...What is the minimum clearance acceptable from the tip of the prop to the hull bottom on a skeg built boat ? in other words how close can you go ?
 

F/V First Team

Admiral
Joined
Jan 10, 2012
Posts
6,146
Likes
2,481
Location
Narnia
Website
www.otisenterprisesmarine.com
Boat Make
Northern Bay 36 - Modified
attachment.php


As close as you dare
 
Last edited by a moderator:

kmac

Commander
Joined
Mar 19, 2012
Posts
352
Likes
100
Location
Chesapeake Bay
15% is the recomended minimum. 20% is better. You can use a prop with a greater E.A.R. or DAR or a 5 blade prop of the same diameter to get more bite---same effect as larger diameter prop. If your slip isn't to great with the present prop you can go up in diameter with more power.
 

Powderpro

Vice Admiral
Joined
Dec 3, 2011
Posts
3,988
Likes
3,863
Location
The Ocean
Website
www.oneilboats.com
Boat Make
O'Neil
Not sure this question belongs in this section but here goes...What is the minimum clearance acceptable from the tip of the prop to the hull bottom on a skeg built boat ? in other words how close can you go ?

I've heard 15% suggested as a general rule of thumb, but I've never had that much clearance on my boats. I try to stay in the 11%-13% range from the bottom of the hull, and it has worked great for the boats I've built. So currently the 22" prop has 12.5% tip clearance. I guess you could jump up to a 23" prop and have 9.8% tip clearance, but I would probably stay with the 22" diameter and go with a skewed 4 blade like the DQX. What kind of hp are you thinking of upgrading to and what does the boat weigh?
 

captainlarry84

Rear Admiral
Joined
Sep 13, 2011
Posts
1,295
Likes
238
Location
Freeport NY
Boat Make
KristenFormer Charter Captain
You can cut pretty close on the skeg as shown by F/T. However you do need a little room on top. Water bouncing off the bottom creates all thoughts of problems. Cupping when clearance is tight helps as it slings the water away from the bottom. Lastly when things are too close it is easy to get debris suck between the wheel & bottom. On DE boats with some cup a close #s is 1.5"s.
 

Rockasauras

Captain
Joined
Nov 9, 2013
Posts
856
Likes
298
Location
OCEAN CITY
I've heard 15% suggested as a general rule of thumb, but I've never had that much clearance on my boats. I try to stay in the 11%-13% range from the bottom of the hull, and it has worked great for the boats I've built. So currently the 22" prop has 12.5% tip clearance. I guess you could jump up to a 23" prop and have 9.8% tip clearance, but I would probably stay with the 22" diameter and go with a skewed 4 blade like the DQX. What kind of hp are you thinking of upgrading to and what does the boat weigh?

Powderpro,

Currently this boat has a 370hp Yanmar and is a 31 footer 12' beam and displacement is 11500. The wheel if I recall is a 22" 3 blade and pitch is 19 (not 100% certain on pitch). I would utilize this motor for the first season and tweak the wheel to try and increase the performance a shade. I was told she cruises at 15-17 kts. 80% load at 17kts. I am wondering if I make a motor swap could I squeeze a QSB 425HP in that space. My goal would be to get this rig to slide along a bit more effortlessly at 19 maybe 20kts with a bit less load percentage.

I will fish the boat only...recreationally (on a fairly regular basis) 300 plus hours a season. I have good confidence in Cummins for two reasons. (1) being I have owned boats powered with them and (2) is the dealer which I have a good relationship with is 1 block from the marina. He is on vacation now in Florida so I have not run these scenarios past him as of yet.
 

Downeast Deadrise

Lieutenant
Joined
Oct 17, 2017
Posts
81
Likes
68
Location
Annapolis MD Severn River
Boat Make
Duffy 35 "MATINICUS" CAT 3208TA 375
Gent's I am batteling with a vibration issue on my Duffy 35, Rockasaurus you know the boat, and I am swinging a 26" x 22 " 4 bladed wheel. Power is a 3208 375 Hp and the current prop runs in the lee about 25 rpm at WOT. If I stand over the prop while running at anything over 2150 RPM I get a distinct buzzing/vibration/tone localized over the propeller section of the deck. I currently have 4 1/2" of tip clearance from the hull which works out to be 18%. I am considering changing wheels to a 24 square to increase the tip clearance to 25% with the hope that it would smoothe out the vibration/buzzing/noise I am trying to avoid. So the question really boils down to whether I am chasing the impossible or if another 2" of clearance will make a noticeable difference. Perhaps taking a little pitch out of my current wheel and adding some cup would be another solution. I like the efficiency that comes with the added diameter. Thoughts?
 

Rockasauras

Captain
Joined
Nov 9, 2013
Posts
856
Likes
298
Location
OCEAN CITY
Gent's I am batteling with a vibration issue on my Duffy 35, Rockasaurus you know the boat, and I am swinging a 26" x 22 " 4 bladed wheel. Power is a 3208 375 Hp and the current prop runs in the lee about 25 rpm at WOT. If I stand over the prop while running at anything over 2150 RPM I get a distinct buzzing/vibration/tone localized over the propeller section of the deck. I currently have 4 1/2" of tip clearance from the hull which works out to be 18%. I am considering changing wheels to a 24 square to increase the tip clearance to 25% with the hope that it would smoothe out the vibration/buzzing/noise I am trying to avoid. So the question really boils down to whether I am chasing the impossible or if another 2" of clearance will make a noticeable difference. Perhaps taking a little pitch out of my current wheel and adding some cup would be another solution. I like the efficiency that comes with the added diameter. Thoughts?

That should certainly be adequate clearance. My experiences with not enough clearance could be described as the sound of gravel hitting the hull when powering up. I have seen the 24 square suggested as the go to wheel for that power/boat countless times. Not so sure I’d go that route yet. Maybe you could find a loaner wheel to try first before taking the plunge ?
 

Powderpro

Vice Admiral
Joined
Dec 3, 2011
Posts
3,988
Likes
3,863
Location
The Ocean
Website
www.oneilboats.com
Boat Make
O'Neil
4 -1/2” of tip clearance is plenty, more than enough for a 26” wheel, I would not reduce the diameter. I’ve had 26” props with 2-7/8” clearance run very smooth, so in my opinion your problem is not tip clearance. Is your current prop, bearings, shaft, engine mounts, and couplings lined up and in perfect condition? Is your rudder bearings/packing all tight and steering ram tight?
 

c1steve

Captain
Joined
Jan 30, 2017
Posts
779
Likes
276
Sometimes a user will install a shaft coupler "out of the box". These will not run perfectly true. First I would check the alignement, and make sure the weight on the mounts is the same, port vs starboard.

Next I would pull the shaft and prop, and have a reliable shaft company check the shaft and face the coupling. Also have them, or a specialty shop, fully analyze the prop.

As Powderpro mentioned, clearance is not your problem.
 

Downeast Deadrise

Lieutenant
Joined
Oct 17, 2017
Posts
81
Likes
68
Location
Annapolis MD Severn River
Boat Make
Duffy 35 "MATINICUS" CAT 3208TA 375
4 -1/2” of tip clearance is plenty, more than enough for a 26” wheel, I would not reduce the diameter. I’ve had 26” props with 2-7/8” clearance run very smooth, so in my opinion your problem is not tip clearance. Is your current prop, bearings, shaft, engine mounts, and couplings lined up and in perfect condition? Is your rudder bearings/packing all tight and steering ram tight?

Thanks for the insight. Mounts are new as is rudder packing. Cutlass and stern bushing are tight and shaft runout looks ok. I have an old stethoscope and I will try listening at different places to see if I can better identify the source. If its prop noise, I would expect to hear a vibration/pulse from the prop each time the blade passed the deadwood. At 2400 RPM/1.98 gear x 4 blades = 4,800. This number may also have to be cut in half based upon the fact that two blades are in the shadow of the deadwood simultaneously during a rotation so the frequency may be 2,400. I just stumbled upon an iphone app that measures vibration and perhaps that will help. If I see a strong vibration in the 2,400 or 4,800 Hz range then the noise is most likely propeller related. Does this make any sense or do I sound insane?
 

Downeast Deadrise

Lieutenant
Joined
Oct 17, 2017
Posts
81
Likes
68
Location
Annapolis MD Severn River
Boat Make
Duffy 35 "MATINICUS" CAT 3208TA 375
What about the condition of your prop? Is it perfect?
I sent it out last year for a tune up and some propspeed. According to prop scan it is right on the money. The engine was originally hard mounted with a straight exhaust. I pulled the engine this past winter and soft mounted it with iso-flex mounts and custom Soundown exhaust system. The new system is so quiet I am now hearing noises that I assume have existed for years but were masked by the engine's structure-borne vibration noise. Glad to hear the consensus is that tip clearance is adequate as it rules out another variable. I'll check the shaft alignment again as well as for rudder bearing chatter and report back. I have a feeling that I am chasing the unattainable with regards to making her run like a modern production boat but I don't see why a DE boat couldn't be made to do so.
 

Downeast Deadrise

Lieutenant
Joined
Oct 17, 2017
Posts
81
Likes
68
Location
Annapolis MD Severn River
Boat Make
Duffy 35 "MATINICUS" CAT 3208TA 375
Now that is something I have not seen before. Looks simple enough to do. Height and width of the wedge as well as it’s fore and aft placement look like critical dimensions. How did you decide on where to place yours? How high above the waterline did you place the air intake hose?
 
Top Bottom