Bought a 2003 Merry Fisher 635 last year. Has an 85hp Nanni 5-250tdi engine, twin disc gearbox with 2:1 reduction ratio, 16" * 14" prop.
I love the boat, but can only get 10knots at WOT (2800rpm) Everyone tells me I should be getting at least 16knots
engine is in great condition (650 hours). Prop is new and only slightly bigger than manufacturers recommended (15"*13") recent haul-out with bottom antifouled and is clean as a whistle. only me in the boat and everything cleared out (tools/oils etc)
I don't know anything about the MF 635 or the Nanni engine. I assume you are talking about a 16" dia and 14" pitch prop with 3 blades. I calculated the propeller slip with the numbers you provided and its .38. I'm guessing at a speed of 10 knots you are plowing through the water, certainly quite a bit faster than hull speed and not nearly planning speed. So the high slip ratio of .38 may make sense. 16" dia is a very large diameter for an 85HP engine. It is a diesel with presumably a lot of torque but its still a big diameter. The difference in power required to turn a propeller going from a 15" to 16" diameter and in addition from a 13" pitch to a 14" pitch is significant. My guess is this engine does not have enough torque and HP to turn this prop at a sufficient RPM to get the hull up on plane. The smaller propeller might be different enough to get up on plane and thereby gain quite a bit of RPM and speed. And of course if your boat is overweight that would be a factor also.
Hi Harry, Welcome to the forum. I am sure you will find lots of good information here and friendly people to answer questions.
We also look forward to hearing about your adventures with the boat.
As a sailor, I don’t have much idea of what speed you should expect, though I agree at 10 knots does not sound like a reasonable maximum, however my experience in industry involved a lot of matching motors to loads
I suspect like Captain Lyn, that the boat is ploughing through the water, making a huge wake bow in the air and chewing up lots of fuel. That is the boat having exceeded its comfortable displacement speed, caught up with its bow wave and trying to climb over it. It has to do this to get on the plane, but it takes a huge amount of thrust to get onto the bow wave and start planing.
The place I would start is by looking at your engine specifications. What rpm is specified at the maximum quoted power, and the quoted maximum rpm, should be two different figures. There may even be a third one, a maximum continuous rpm, something lower than the maximum rpm.
Then look at your tacho. (I am assuming you have one, otherwise it should definitely be your first upgrade. It is basic to understanding how your engine is performing, not just for this issue, but in the future as well). At wide open throttle, your engine should be able to reach that maximum specified rpm or a little more. I am suspecting that the maximum specified rpm is somewhat more than your 2800 rpm, and that difference is important. It basically indicates that your engine is overloaded as Captain Lyn has indicated.
If you reduce the load imposed by the propellor, the engine will not only rev higher but also produce more power, which is what you need to get over that bow wave, and continue planing, with still some power to increase to maximum boat speed on the plane.
With a fixed blade propellor, this does involve a change of propellor, and hopefully your dealer has a few and will help you try a few different combinations to optimise the selection. Of course, as a preowned boat, the history of how that prop got there is not really known, and understandably a dealer may not be able to help. However, if you go along to an independent propellor manufacturer, they will be able to make a reliable check of the selection, and with a bit of luck, help you through some trials as part of purchasing the right one.
All that assumes the engine is in good condition, properly serviced clean air filters and so on, and that the engine compartment is well ventilated to provide good air flow, just the normal issues to check that the engine is ok. If you are unsure, an inspection by a mechanic might be a good investment anyway, as breakdowns on the water tend to spoil your day. Also I am assuming your hull is nice and clean, and no dings in the propellor blades, as they cause an enormous increase in drag on the propellor, out of proportion to their appearance, weight properly distributed for a well trimmed hull.
I hope this is of some help, and look forward to hearing how you go.
thanks to Capt. Lynn and Rene for your posts and your advice is much appreciated.
Yes, the prop is 16"dia and 14"pitch, three blade. The engine is rated and governed at 2800rpm and it flies up to max rpm easily which makes me think the prop is underpitched.
However, as you both point out prop sizing is a very technical procedure with an element of try it and see!
Yes, the boat is plowing through the water and there is a pretty big wake, so I agree 85Hp is too small to even semi-plane.
After speaking to some of my colleages with similar size boats, but more Hp (around 100-125) they say their boats never actually get onto the plane, but they will semi plane at around 15-20kn
So, taking all your advice onboard I think I could probably get another 2-3kn experimenting with various prop sizes/ratios (which would be a very costly exercise, with lift outs as well) - but is it worth it as I'll still be displacing water. Probably not.
My colleages advise, save your money, run at 10kn, but leave a day earlier than us, for our group fishing trips!
Hi Harry, the rated speed of 2800, reached easily tends to confirm your feeling of not enough pitch rather than overload which was my first thought. And your colleagues description of performance with larger motors suggests that the 85 may have been selected more for fishing at trawling speeds or river cruising than for a fast run out to the favourite fishing ground.
It may not be worth chasing that few extra knots if it is not going to get you onto a clean plane, and if you don’t have a trailer to make haulout easy. I had assumed the 695 is a trailerable, so haulout would not be an issue if a local prop supplier would help you try a couple. The specialist prop suppliers can apply quite good technology to come up with a pretty good selection first up, so could be worth getting a recommendation for your boat and engine, and considering a purchase ready to change at next haulout. Especially if they would take back your old prop in part exchange.
However, may I make a few suggestions about operating in that “ploughing” mode. I don’t know if you are in open waters or not, but that wake is not without consequences to other boats around you, or to river banks and lake edges. And the wake travels a long way in smooth water.
So may I suggest you try a couple of tests. Starting with full fuel tanks, try similar runs of decent distance first at your ploughing speed, then a bit slower so as to leave minimal wake. You will notice the difference in noise, and feel of the boat, but fill up again after each run to check your fuel consumption. And see if you have a fuel consumption quoted in the specifications. This will indicate whether you are using your full 85 hp or operating part throttle due to the governor. Diesels need a good load for long reliable life, and certainly my Yanmar manual recommends that if you must operate long periods at low load, that you race the engine is neutral a couple of times each hour. I would assume the Nanni manual has something similar.
Please don’t take this as advice, just an attempt to discuss the issues around your initial question with like minded boating enthusiasts, to aid understanding of what is going on.
many thanks for your posting, I appreciate the time and effort that goes with your reply and I agree with all your comments.
it's not worth chasing that few extra knots at this time. Next haul out maybe, and by then I'll have a better knowledge of the optimum prop for the boat.
I also agree with your thoughts on "plowing". In my location we are in a river estuary with a 6kn speed limit which gives minimal wake, then out to sea with no limit of course. Even at sea, I don't like running the boat continuously at max 3000rpm (10kn) because, as you indicate, its probably the most inefficient speed to run at. I therefore generally run at about 2100rpm (6kn) which is a comfortable cruising speed for me, the boat and the engine, and 3000rpm on return to port depending on sea conditions.
However, when I bought the boat last year I did expect that 16kn would be the max (semi planing) and 6-8 kn would be my happy cruising speed, as I don't need to go flying around anymore. It's too late to do anything about that now, so will keep the boat for this season, enjoy it and decide what to at the end of the year.
It's been good to get a perspective from a fellow boater and appreciate all your help and suggestions. i hope you have a good season.