Prop sizing – diameter and pitch

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    Hi all,

    We are about to replace the prop on Rainshadow. It’s currently 16″ x 12″ and badly corroded. Marilyn started a thread a while ago asking about feathering props, etc. This is thread it mostly about prop sizing and pitch. In the first thread, Jeff described his experience after repowering and putting on an 18″ x 12″ prop. However, because of the different shaft HP and reduction ratio, it’s not clear how his results apply.

    I worked through the prop calculations (per Nigel Calder, and other sources) to calculate the correct prop pitch. In these calculations, you take several input parameters (eg Perkins 4-108: Shaft HP = 35, LWL = 27 feet, Loaded displacement = 20,000 lbs) and then use a terrible fudge factor called the “slip” to estimate the correct pitch.

    I say “terrible fudge factor” because it depends on the hull shape, smoothness, etc, so it’s not known a priori – but it should be similar for the same model boat. Fudge factors quoted in the literature vary by large ranges (20%) – which translate into large ranges in suggested pitch.

    When I run the numbers, I get a pitch of 9″-10.5″. So the obvious question to all you Nic 38 owners out there (with the Perk 4-108 and the Thamesway tranny – 2:1) is = what is the pitch of your prop, and how well does it work? Can you get up to 3000-32000 RPM? Can you reach hull speed @ 7kn?

    We have the as-recommended in 1974, 11.5″ pitch, and cannot run up above 2000 RPM before terrible vibration and cavitation occurs. Is this because we are over-propped with the pitch too large? We don’t know for sure because our prop is in poor condition – badly corroded and pitted, which might lead to premature cavitation (and maybe vibration).

    So, what’s your prop pitch and how does it run – what’s your max speed and RPM? If you have a graph of speed vs RPM, that would be even better!



    Arild Jaeger

    Hi Van,

    Our Nic 38 has a Perkins 4.108 (which I believe is 49 hp) with the TMP 12000 gear and the old 16″ X 12″ three bladed propeller. We normally cruise at 2000-2100 engine rpm giving us about 6.5 knots. I think the max rpm is in the region of 2800-2900 taking us to 7.5-8 knots. I probably have a diagram somewhere which I will post later. My propeller is probably from 1968 and is pitted to the extent that there are some small holes through the blades. Still, I have no plans to replace it.

    I think the pitch and diameter are well suited to the boat.

    The propeller has a very uneven inflow of water from the front side with a short distance to the relatively wide deadwood. This will reduce the efficiency of the propeller. Also the distance between the propeller blade tips and the hull is very small, which means there will be a lot of noise and vibration. To reduce the noise you could reduce the propeller diameter to increase the tip clearance, but this would further reduce efficiency. You could also increase the number of blades on the propeller and use scewed back blades, both these measures would reduce the impact on the hull of each blade passing and thereby the noise and vibrations.



    Hi Arild,

    Thanks for the info.

    Very interesting comments about your speed vs RPM. Since you can throttle up to 2900 RPM (which is only 10% or so from the nominal max at 3200, and which is easily within the errors of the tachometer) it sounds like the prop is not overpitched much if at all. It does seem about right.

    BTW, the 4-108 is rated at about 50 BHP (brake HP) which means running on a test bench under ideal conditions with no gearbox, alternator, etc. The HP at the shaft, so called SHP, is supposedly only 35-40 once you add all the other stuff.

    Thanks again!


    Arild Jaeger


    I found my old test data, which show a maximum RPM of 2800 giving a speed somewhere between 7.3 and 8.0 knots (GPS / Sumlog). I don’t know how to upload the excel file to this site, but the key data are:

    Engine GPS Sumlog
    RPM Knots Knots
    890 3,1 2,0
    1 090 3,7 2,8
    1 350 5,5 4,4
    1 700 6,0 5,8
    2 010 6,6 6,7
    2 250 6,8 7,1
    2 500 7,0 7,7
    2 800 7,3 8,0

    We also measured the bollard pull at different engine RPMs:

    Engine Bollard pull
    (R.P.M.) (kp)
    890 45
    1000 60
    1100 68
    1200 84
    1300 95
    1400 115
    1550 136
    1750 180
    1910 212
    2070 250
    2260 295
    2410 340
    2500 365

    Finally we tried to measure the towing force at different speeds (not very stable results, but a good indication when you plot the results):

    Speed Resistance
    (knots) (kp)
    2,0 32
    2,0 38
    2,4 50
    3,3 60
    3,6 40
    4,5 148
    4,6 90
    5,0 70
    5,5 100
    5,8 100
    6,2 150
    6,2 100
    6,2 125
    6,4 180
    6,5 200
    6,5 200
    6,6 150
    6,7 200
    7,2 210

    If someone can help with uploading the spreadsheets, you will also see the related graphs.

    Kind regards,


    Arild, thanks for this great data. I reran the numbers and here’s what I got.

    I assumed the actual hull speed is about 8 knots per your data (remember the old 1.34 * sqrt(LWL) is just an approximation that depends on hull shape, etc). I also extrapolated your data a bit to estimate that at 3200 RPM you’d hit that 8 knots. The slope of the plot of your RPM vs speed data does imply it can reach that.

    Anyway, I also assumed the Shaft HP at 42HP.

    The theoretical diameter works out to be 16.0 inches, nicely matching what C&N used!

    The “slip” works out to be 49% for your 12″ pitch prop. This is at the upper end of the range typically quoted (40% – 45%).

    The interesting thing, is that the calculated optimum blade area is 186 square inches. For this area, for a 16 inch diameter prop, the recommendation is a 4-bladed prop with wide blades. Apparently, using the three blade, with smaller surface area, will lead to more cavitation at the higher RPMs. This may also explain the slightly higher slip numbers.

    One last thing, we are considering getting a Featherstream feathering prop. The manufacturer ( claims to have installed these on at least one Nic 38 – does anyone reading this know anything about them? They claim it fits in the aperture.

    Thanks again for the data, Arild.



    Van and I finally reached a solution for the prop on Rainshadow – the Axiom. Van has prepared detailed reports on our blog,
    Part I: analysis of the correct prop sizing
    Part II: The Axiom installed
    Part III: The second Axiom installed

    Short version is – we like our new Axiom prop. I need more time at the helm getting used to it, but it’s certainly an improvement over our 16×10 corroded screw!

    As we say in the blog postings, Axiom the company was excellent to work with – they stuck by their promise to get us a prop that works. You can find them online at


    Evening all,
    I’m following this discussion very closely as I was less than happy with my replacement prop. I dropped any further investigation at the time as I had other things to spend money on, but the 18×12″ was considerably better than the old 16×11.5″(which was & is still in almost 1st class condition)But 18″ was just TOO close to the hull. At low speed, i.e, in harbour or marina the propulsion & control was very impressive, but at above 2000rpm the noise & cavitation was horrendous & I couldn’t get her even close to 3000rpm. I sent the prop away & had it cut down to 17×11.5″ with all the outer edges chamfered & the rpm improved to over 3,500. Trouble is, she still won’t go any faster than 6.0 knots. Whether that’s down to the prop or the fact that I have 4x 255 AHr Lifeline batteries weighing over 250Kgs in the engine compartment,which I’m sure affects the trim somewhat, I’m not sure. I’m beginning to regret buying so much battery capacity, but the received wisdom at the time, on the forum, was that for long-distance cruising one should have as much as possible, but in my opinion the shape of the Nic-38 is not well suited for this being fairly fine at the ends. If she carried her beam well aft, like more modern designs, she could carry more weight without disturbing the trim very much, so I’m thinking of selling off at least 1 of the Lifelines to see how much difference it would make.
    I like the idea of a 4x bladed prop though, I wonder which Nic-38 it was.? Be very interested to speak to the owner.!


    Hi Jeff –

    We do not know of a Nic 38 with a 4 blade prop. The comment about such a prop in our blog is that we considered buying one based on the calculations, but the closest we could find was a 17″-10 4 blade cut down to a 16″. Our calculations suggested what would be appropriate is a 15″-10 4 blade.

    You mention when you had the 18″ screw you had great control in marinas. I experienced the same with our first Axiom that came too close the the hull. In fact, I had too much power at idle RPM gearbox engaged for my preferred slow speeds in tight quarters – but I got used to it. Now with our new Axiom prop, I feel almost nothing at idle RPM gearbox engaged, so I need to get used to giving it some throttle to get us moving. Once I start doing that, I might feel like I have a little more control in reverse with our second Axiom. (Van has a better opinion of the steerage control it provides in reverse than I do, but he points out that maybe I expect too much.)
    Marilyn and Van

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