Getting more from a 70 hp Johnson on a Crestliner 1750 Fishwawk

[dadoe]

I attempting to buy a 2000 1750 Crestliner Fish Hawk with a 70 hp Johnson 2 stroke. The boat is rated for 115 Max and I see most equipped with 90 hp. My guess is this boat goes no faster than 35 mph with this set-up. I would like to see a 5 to 10 mph increase if possible.

Question: How can I increase performance without sacrificing motor integrity? What would be a good SS prop to buy?

Keep in mind that I am outboard stupid.

Thanks


Modified by dadoe at 12:16 PM 10/19/2005


Modified by dadoe at 1:18 PM 10/19/2005

[astrofsx]

I have a good friend with the 1750 fishhawk with a 4 stroke 90hp. He was having a huge problem with the boat porpoising no matter where the weight was positioned. I know part of his problem was the weight of the 4 stroke, but i have heard many people with this boat with the same problem. We were at 37-38 mph with no usable trim before porpoising. We tried to move weight around, move the motor up and down, and put a stabilizer/hydrofoil on the motor and the only thing that cured his was a set of Nauticus smart tabs. Now the boat doesn't porpoise near as soon and he is able to actually use the trim. He is now running about 42-43mph at best. He is currently running an aluminum prop and is saving up for a stainless one. The only stainless prop that he has used was a 20 pitch high 5 which was slow and a dog out of the hole compared to his aluminum 19 pitch hustler prop. As far as choices of stainless props, one of the gurus on the board will be able to give you a much better recommendation than me, but if you find one that works great, please post it so i can let him know.

Steve

[Earl320V]

Paul: you could try several things but there not going to be cheap, 1) get a water pressure guage installed in the boat 2) try a jackplate I would think a 4 " 3) I would get a 18 pitch Raker, with this being said I know you won't see a 10 MPH increase, maybe 3-4 .. The jackplate allows you to raise the motor higher out of the water,but you need the pressure guage so you don't raise it too high and burn it up. Its a heavy boat,underpowered ,good luck....

[Earl320V]

Paul, I know where a 18' Ranger is,has a 150 Merc on it, its a older boat,looks good from the road I don't know what the guy is asking for it,just another possible lead for you,and its a dual consol..(Sp)....

[Jason Phillips]

Please don't take this the wrong way, I just want to head off a question before it is asked. We do not do motor hop-ups on this board. So, if you want to know how to get more than 70 hp out of your motor, you won't be able to get that information here. We can help with setup and offer suggestions that will assist you in getting the best performance from what you have, but we will not post things to increase the power of your motor and thus cause less reliability. We want all motors to run for as long as possible.

Your biggest gain will come from the correct prop.

[dadoe]

I'm not looking for tips to "soup up" the motor. I stated that I do not want the integrity of the engine impacted in a negative way. The only trick that I know is to improve the prop.

Another suggestion mentioned was a jack plate. I'm grabbing for straws hear but maybe a better spark plug and air cleaner would help like it can do for vehicles. I don't know what my options are and I thought I can poll the membership for ideas. I am open to the possibilty that I have no options, also.

[Jason Phillips]

Like I said, don't take that the wrong way....

An underpowered rig is difficult to work with. The motor is having to strain to push the boat whether it appears that way to the owner or not. Being an older motor, a good tuneup may do wonders to how it runs. A jackplate allows you to adjust the height of the motor very easily and also gives the motor additional setback which can help holeshot as well as top end slightly on most performance boats. The prop would be a major area that you could see improvements in.

With all of this considered, you need to look at where your money will be best spent. A couple hundred or a tuneup. A couple hundred for a stainless prop. A couple hundred for a jack-plate. With these things and having the boat set up properly, you will probably gain 3-4 mph tops. So then the question has to be "Would I rather spend this money on something else, such as equipment, and just enjoy how the boat performs?"

[dadoe]

Very good point and I never thought of it like that. I did not take your response as off key, I wanted you to know tht my intentions were "pure".

In general, how would I know the difference between underpowered and low hp'ed? In other words, if 70 hp is well within the manufacturers recommended hp range would'nt it be fine under normal load, just not as high performing as higher horse power.

I'm really new to this stuff that is why my questions must seem stupid to most of you. The answers have been a big help, though.

It looks as if a serious test drive needs to be in order.

[Jason Phillips]

None of your questions are stupid.

Your motor will have a recommended operating range. Let's just say 4500-5800 rpms. If you can only get that motor up to 4700-4900 rpms, you are still in the recommended range, but the motor is lugging at that point. 2-strokes like to run in the upper end of their power curve. It keeps things clean inside. If that's all you can get, then you'd need to change props to try and get your rpms up. If you can't get them up, you are underpowered too much. If the motor runs good, seems to push the boat ok, but takes a while to get up to the 5500-5700 range, then you may still be underpowered, but it is useable. A good tuneup may help with the pep to get you up in the rpms faster. Otherwise, it's just a drive it and enjoy it at that point. That would just be a low horsepower situation.

Does that make any sense?

[dadoe]

Believe it or not ....perfect sense. My focus was on mph not rpms. RPMs is key. Thanks for the lesson. Should I get this rig then I'll report my findings.

[61Woody]

Dadoe, you are right about 35 MPH that is all your going to get out of that 70, don't waste time with a jack plate as the way the crestliners are shaped it won't do much good anyhow. I have a 1600 with a 75 that get low 40's and porpoisis alot with very little trim, going to experiment with props again next year. BTW call Crestliner and they will tell you what speeds their boats run with various motors.

[Earl320V]

I agree with Stratos,but have a different opinion than Hawkeye's I (with in) reason feel that a jackplate will help most boats one can fine tune the engine to get better(more) RPM's out of the motor,basicly the more RPM's you can get the fast the boat will be. But when playing this game it can become a expensive ,trial and error.I still think your way underpowered for that hull....