Questions on propellers

Nick Zito

Nick Zito

Active member
Since I know there are folks out here that know more than I do, and it may save me time to ask you all as opposed to scouring the inter webs, I had some questions on props.

I have a '98 Suzuki DF40. It's spotless inside. Guys I bought the boat from didn't know much about keeping a boat in perfect condition, but they did with motors. Anyways, my DW15 is rated for 4 persons or 1600lbs with gear. When I run with 4 people and gear, it struggles to get on a plane. Takes a while, even with two of the heavier folks in the bow. Once it gets up there it's fine, but could take 5 minutes or so. Now granted I may be fighting the current and I know we're probably pushing the limit on what the 40 will take.

Last week I added a trolling motor and subsequent heavy deep cycle battery to the stern. With two kids (maybe 50 lbs each) and two adults it did take a bit to get on a plane. Went easily when I had Andrea move more to the front as well.

Anywho, with two adults and gear it'll fly at 35mph. Honestly a little scary for me growing up on a sailboat. I assume I can change the prop pitch and whatnot to give me more oomph and less speed. Would be happy if top speed was even 25 if I could hoof it to plane faster.

Hoping someone can explain this to me fairly simply.
 
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Nick,

Have you tried trimming the motor down more? Does it have power tilt/trim? If so, can can trim all the way down to help get up on plane and then trim up as needed. If no power T/T, you could just try it in a lower manual position.

Not sure about propeller.

Matt
 
Nick,
The pitch refers to the theoretical distance the prop will advance through the water with one full revolution of the prop. Using a prop with a smaller pitch can be viewed somewhat like using a lower gear on your bicycle or car. A lower gear (lower pitched prop) will probably get the boat on plane more easily. More power but less speed. Some caution must be used in choosing a new prop. If the pitch is too little, the motor can be run past its maximum safe RPM range, possibly resulting in a blown engine. Too much pitch, on the other hand, can result in lugging the motor, preventing it from getting up into its strongest powerband. Everyone will tell you to use a tachometer to check the motor's RPM at wide-open-throttle. If the prop is properly sized to the boat/load, the top speed should fall within the published top end range of the motor. They're not wrong, but that doesn't really cover all the possible conditions. Personally, I chose a prop with the smallest pitch I could easily find at the time so I could get my heavily-loaded layout rig on plane. In summer fishing season, I simply avoid running the boat at or near wide-open-throttle so I don't overspeed the motor. Suits me fine.
 
You need a lower pitch, likes been inferred, whatever you have now is listed on the prop, the second number is the pitch, going down one size should give you about 150- 2something more rpms...ideally a tach will show you where you are, with a normal load you want to be about top rpm for your motor, that's listed in your manual, but is usually around 5200 - 5500 rpm. If your only doing lets say 4800, your losing planing ability.
What you don't want is to over rev the motor, but lugging the motor isn't good either. That's why the tach is important. I believe your model will accommodate a temporary tach, thats how the dealer would determine what prop is best. But one size drop shouldn't be an issue with what your experiencing.
 
I don?t think you should be surprised with a four stroke 40 struggling to get on plane with 4 adults. If you can get on plane just be glad with that load.
 
I have power trim and tilt, and it's not going any lower. WOT is about 5500, but I never run it that fast. I noticed the lagging when trying to get on a plane, it just kind of sits at 4200 until something changes enough that it starts to plane and then it flies up to 5500, but I typically run at about 4200 once planing and cruise at 18-20mph depending on current and wind.
 
I should add that I didn't mind much when it was lagging with 4 adults and a kayak strapped to the bow and the 3 dozen floater goose decoys. However it was a little concerning with two adults and two small children (and the 100lb deep cycle motor) dragging behind. I'd rather get on a plane quicker and run slower like stated before. If my top speed drops 5-10mph I could care less.

I should add there is a tach on the console so it's easy to watch.
 
Nick ,
Im no expert but If you can reach 5500 rpm with a full load I would say that the prop is sized correctly or maybe a little light on pitch. I was always told you are propped correctly when you can hit the motors max operating rpm with a light load, seems like you can get there with a decent load. . To me it seems as the boat is just heavy and needs some extra umph to get it on plane as when you hit planing speed you can rev right up and possibly back it down and keep it on plane. Maybe to much drag from the lower unit. You should look to see where the motors cavitation plate is in relation to the bottom of the boat. Maybe raise it a hole. Also have you ever weighed the boat and motor? Boat could have some waterlogged foam! Just a thought. Sounds like that 40 is doing all it can.
 
Yes, once it's on a plane I can move the rpms wherever I want, but I like to cruise at 4200 and it's associated 20mph. It will stop planing when I get below maybe 3650 (after I've already started planing). With two adults it gets on a plane no problem, with more than that it does struggle a little, especially with the heavy battery in the rear now.

I'll post some pics of the motor down vs the hull of the boat so you all can take a look. Maybe I just grab a prop With a slightly lower pitch And see what happens. Won't be home until tomorrow so I can't even check what's on there now, but am curious.
 
I have a fair bit of knowledge on props coming from the offshore fishing size. If you have a way to measure your actual RPM: You should be roughly 2-5% over max stated RPM's at a full load. This is the rule of thumb for marine diesels. For example, my boat which has CAT C9 ACERTS is stated to have a 2500 max RPM, electronically governed at 2550. The goal is to hit 2550 fully loaded, but at least 2500. My right motor hits 2550 and the left hits 2535 (higher load on that side, reduction gearset).

For your purposes, just make sure you hit max RPM fully loaded. If you do, you're golden. Anything less is going to put premature wear on the motor and gearcase.
 
T Entwistle said:
Nick ,
Maybe to much drag from the lower unit. You should look to see where the motors cavitation plate is in relation to the bottom of the boat. Maybe raise it a hole.
Click to expand...


Nick,
The photos seem to support the above statements. Hard to tell for sure from the photos due to the camera angle but it appears the motor is sitting lower than optimal.

Also one must be aware of just what angle the motor is trimmed to, while attempting to get on plane. Trimmed out too much and the back end of the boat is driven down into the water and the thrust is directed towards the waters surface. Trimmed in too much and the bow is driven deeper into the water and just plows a large bow wave, that it must overcome to get on plane. Trial and error will teach you the sweet spot for proper trim. You most likely will find that the engine needs to be at one certain trim angle for getting on plane (most efficiently) and a different trim angle to stay on plane (most efficiently).
 
I believe the motor is sitting slightly lower than what you would want but in no way do I feel it is too low. I agree with the trim comment though. It does look like it is trimmed back a little too far.
 
Thanks T. Will fiddle with the motor later today when I get back home. Opening day tomorrow means lots of prep for the backwaters tomorrow
 
If it?s power trim and tilt you did not have to move that. If it?s not give it a try. It will just limit how much negative trim allowed. I was talking moving the complete motor up one hole so the cavitation plate on the motor is more inline with the bottom of the hull. According to your statements before I think you are propped properly. Just have to fine tune the motor location and trim position. Even on my 23? with a 250 when you load it up with gear,decoys, guns and layout it takes more time to plane. Every boat responds differently to trim adjustments. You just have to learn where it likes to be under certain conditions.
Good luck tomorrow. No boat for me in the morning. My woodie spot is a little to shallow!
 
Ditto. I rebuilt the switch plate on this and haven't installed yet. Woodies and geese tomorrow in the kayaks. Won't be using this until regular season.

Currently the motor mount L bracket sit on top of the transom. If I lift it up one hole it will only be held to the transom with the 4 bolts, no L bracket. Does this pose an issue? I can send a photo if needed for clarification.
 
In an effort to clear up some terminology:

The motor's cavitation plate looks fine. It seems to be about even with the lowest portion of the hull down the center. I believe that's fine.

The motor's trim on the boat, looks to be trimmed too far down or a little low. That's where your problem may be.

I believe we're all saying the same thing, just different terminology.
 
You want that plate your pointing to even, or just a half inch lower then the bottom of the hull.....you have more drag then you want.
 
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