300 Johnson prop

[surprise]

I fish offshore with a buddy who has a 25'Hydrasports Vector WA with a '90 300 Johnson. The motor runs well, but the boat speed and fuel use are disappointing. WOT is 36-37mph at 5700, this is @about 1/2 tank of fuel and two people onboard (although admittedly with a lot of fishing gear--probably another 300 lbs.) Cruise is 25 mph @4700. The boat struggles when the seas are up, going maybe 18mph @4000. It gets maybe 1.2 mpg., worse if conditions are less than ideal. It currently has a 14.5"X19 3 blade SS prop. A prop man has suggested a 15.5"X17 Mercury Mirage SS 3 blade prop. Opinions?

 

[WillyBWright]

Re: 300 Johnson prop

I like that suggestion. Their hub system makes them mountable on most any brand. The Mirage prop produces Stern Lift which can help alot with holeshot. Lowering the pitch also sounds logical as long as he's not exceeding the maximum RPMs at full throttle. If he doesn't want to spend that much money, there is a 4-blade aluminum Alpha prop by Mercury that would also do well. They come in 16, 18 and 20 pitches and produce stern lift. Both are sterndrive props, but an a$$-heavy boat is an a$$-heavy boat whether has a drive on the back or an outboard.

 

[surprise]

Re: 300 Johnson prop

I don't know if the boat needs stern lift, it planes out with almost no bow rise, seems like the prop is just getting a lot of slippage. I have a boat of similar size and weight, but with twin OMC 2.0L (140's) SeaDrives. It also has 14.5X19 SS props and it is faster at WOT (42+ mph) and cruises at 28-29 mph @4000. I figure the V4 SeaDrives have a lower gear ratio than the Johnson 300, so the HydraSport must be suffering from a lot of slippage. My boat with twins, more windage (has a flybridge), and more transom deadrise is much quicker and easier on fuel than the HS, in spite of having 20 fewer horses. I do get plenty of bow rise unless I use the trim tabs, but it planes out PDQ either way. Some of the performance edge may be due to the motors being bracket mounted, but I wouldn't think it would make that much difference. My buddy and I think maybe a 4 blade is the way to go on his boat, I guess we'll see. Just wondering if anybody else on the board had been through the experience of propping a big heavy single outboard boat.

 

[Dhadley]

Re: 300 Johnson prop

On a 4.0 V8, 5700 is only starting to get into the power band. If he's going to stay under 6000, as the target wot rpm, he needs to get a V6. A V8 set up at 5700 is only making a little over 200 hp. Those motors are designed to be set up around 6500. The limiter is at 6700.<br /><br />The prop pitch seems about right. I'd work with the set up to gain rpm's. Keep in mind a "20 inch" V8 is really 23 inches long. A "25 inch" V8 is really 28 inches long. Its EXTREMELY common for V8's to be mounted too low. They have large gearcases that cause a lot of drag. <br /><br />A V8 set up is totally unlike a V6 set up. For some reason that concept is really hard for some installers to understand. <br /><br />I take it from your post that the motor is transom mounted. My guess is that it's probably mounted nearly all the way down. I'd further guess that it's a "25 inch" motor on a 25" transom. If so I'd bet that he'll be able to raise the motor a lot. Probably more than the 4 holes will allow. Its not uncommon to use a "20 inch" V8 on a 25" transom.<br /><br />V8-land is a wonderful place. But its unlike anywhere else youve been.

 

[surprise]

Re: 300 Johnson prop

Dhadley, are you suggesting to aim for 6500 WOT? Isn't the max. WOT spec for these engines 6000? You are right,it is a transom mount, undoubtedly a 25" transom on the boat. I have been wondering if a jackplate would help. But maybe we need to try raising the motor up a hole at a time and see what happens, haven't noticed what hole it is in now. It doesn't respond a whole lot to trimming, so what you are saying makes sense. There does appear to be room for a larger diameter prop and I still think it would do better with something with more blade area on this heavy boat. The motor wasn't even set up to open the carbs all the way when he got it--got 300 more rpm just from fixing that--may have been done because it had a bulb on the fuel line, we changed it out to the electric priming pump. Yeah, the gearcase is a beast, isn't it?

 

[Dhadley]

Re: 300 Johnson prop

The rev limiter is at 6700. I set mine up for that. Youll be amazed how much power is left in the last 1000 rpm. If he isnt going to set it up over 6000, then he may as well get a V6 and save carring around all that weight.<br /><br />Primer bulb huh? See what I mean about riggers not understanding V8's? No primer bulb or 90 degree fittings or bulkhead fittings or any fuel restriction allowed! Make sure you use the factory fittings and copper pipe on the primer pump. And make REAL sure the fuel delivery hose is all 3/8" ID to the first pump.<br /><br />Youll be able to get a large diameter prop on there (good idea) but a 4 blade is probably not a great idea until you are able to get the motor up.

 

[surprise]

Re: 300 Johnson prop

Dhadley, it had the electric pump originally, the button for it was still at the helm, somebody took it off, don't know who. I gave my buddy one of the OMC pumps that I got with my SeaDrives (two motors in a basket for $1500, but that's another story), anyway, the local OMC man says it is the factory set-up, has copper tubing bypass. There is still a 90 degree fitting in the system though, I think where it goes to the bulkhead. Guess we need to re-route--maybe put the fuel hose thru a grommet in the bulkhead? Also, this motor still has VRO's on it, which makes us both nervous--plan is to take them off and mix fuel, any advice on that? We will try raising the motor, will keep you posted, but probably won't try it out 'til warmer weather, it is a long way on the trailer to suitable water for testing. Motor did sound a lot better moving up from 5400 to 5700 @WOT, must sound great at 6700.

 

[Dhadley]

Re: 300 Johnson prop

Yep, I think they sound great. You don't have to remove the pumps to mix -- just disconnect the oil inlet line, plug the oil inlet side of the pumps and disconnect related wiring on the pumps. Those pumps will pump quite a lot of fuel, no problem. <br /><br />Yes, get rid of any and all 90 degree fittings and anything else that resembles a restriction. Including the anti-siphon valve in the tank(s).

 

[surprise]

Re: 300 Johnson prop

Thanks, Dhadley, for the help. I talked to my buddy last night, he may post with more questions. Sounds like we need to get rid of some fittings, I don't know if there is an anti-siphon valve on the tank, will check. I guess fuel restriction be a factor in the rpms @ WOT, so need to deal with that first. He says it is very difficult to get the motor to cavitate, so there is another indication it may be set too low. The cavitation plate is roughly in line with the bottom of the vee, but it seems like it needs to come up some. Now we just have to figure out how to lift the beast. I am thinking some kind of set up using a floor jack, failing that, will use an engine hoist.

 

[Dhadley]

Re: 300 Johnson prop

A hoist is always the best way. You have to remove the top bolts for sure. The motor will slide on the bottom two. We prefer to remove the motor compltely to clean off the old sealer and get a good pattern with the new sealer.<br /><br />Your correct about the venting deal. If you can use all the trim range its probably a bit low for that particular prop. Dont get concerned about changing props though. Lets raise the motor and see how it responds overall. Especially in regards to rpm gain. We dont want to lose the baseline by making 2 changes at once (X dimension and prop).